Big Block Chevy in a G-body!

Pic. 1

Another popular question I receive is "What has to be done to fit a big block Chevy motor into a G-body?". 

I started to go over the pros and cons of choosing a big block, but that is a totally different story.  I'll just assume you have made that decision and want to know what is required.

As with any engine swap, modifications are necessary and expected.  The great thing about the G-body cars is that they have PLENTY of room for a big block!  People that see mine always say, "Hey, that doesn't look like that tight of a fit!". Really, it isn't.   Modifications are actually few and very simple.  No cutting or welding on your car is required, so it can be easily changed back!  You even use the stock small block chevy motor mounts!

I am basing some of the information that I give here on what I have read in magazine articles.  I cannot guarentee the it is the absolute truth, but will take their word for it.  I am open to comments, suggestions and corrections if you have any.....

I may have made it a little easier in my case, since I decided to remove the Air Conditioning and Heater box on the passenger side of the firewall.  This gives tons of room to work with.  See Pic. 1 above.

But, you may want a more streetable setup and maintain (at least) your heater and maybe even air conditioning.  It is possible with no major problem.  You can keep all the accessories including power steering if desired. 

Onto the swap!  

Motor Mounts
The big block drops right into the '78 and up G-body's.  Good news is, you will use the stock small block mounts and brackets (clamshells).  If you have a V6 or non-chevy G-body, you will need to get the following (GM part numbers):  Mounts-459021, Brackets (clamshells)-334970.  I am not sure about location (which bolt holes) of the mounts if installing into a non-chevy application, I will try to get this info. 

At the time I built mine, Hooker Headers was the only company that made big block swap headers for the G-bodies (that I was aware of).  Here are the known headers that will fit:  

Hooker Headers #2241 - 2" primaries, 3 1/2 collectors, doesn't come with reducers (collector exhaust hook-up).   I used Hooker Headers on my BBC swap.  They come with two slip tubes on the passenger side to ease installation if you keep the A/C and heater box installed.  Also, it has one slip tube on the drivers side that goes around the outside of the steering column, see pic. 1 and pic. 2.  As you can see the fit is tight, but they fit really well.  They only modifications required are to shave a corner off the drivers side motor mount (clam shell).  It is a simple mod, and only takes about 2 minutes with a cut-off wheel.  In the pic. 3 below, I drew a line where the corner of the mount will have to be trimmed off.  Also, you may need to swap the bolt around in the passenger side lower control arm to keep the header from being too close to the control arm bolt.  See pic. 4 below for a close-up shot.   Note:  Here's a little tip from my personal experience when dealing with the Hooker 2241 headers.  Before installing the engine, I would recommend dropping the driver's side header into the engine compartment and hold it in place by using strings, cables or bungie straps.  Then drop the motor place.  If you install the motor first, it is a MAJOR PAIN installing the driver's side header.  


Pic. 2

Pic. 3   

Pic. 4


Hedman Hedders #65216 - 1 7/8" primaries, 3 1/2" collectors.  I haven't used these, so all I can give is the application notes that are listed on the Hedman website (  Here they are:  Designed for stock mounts in stock location and stock width oil pan, Under chassis exit, Hedders come with street hook-ups, Must notch crossmember on driver's side, Hedder comes with weld-on collectors, Competition hedder - Allow for extended delivery time.


Valve Covers
I used tall valve covers without any problems.  But, the swap articles I have read, says that you may have some interference problems with the A/C-heater box.  They clear the power brake booster fine, but are tight.  I am able to remove the drivers side valve cover with no problems. 

The good news is there is not really a clearance problem here.  You have plenty of room between the distributor and the firewall, even with a HEI!  You can even use your HEI out of your smallblock if you want!  I use an HEI type distributor, and the only area you will need to be concerned with is the windshield wiper mechanism arm hitting against the plug wire boots.  You should be able to work around this without any problems, by clocking the distributor so this does not happen.  Of course, if the HEI fits, then the small diameter distributors are a piece of cake. 

Water Pump and Accessories
Big block Chevy engines uses two styles of water pumps.  The '69 and up models use a "long" waterpump, which is the one you will want to use.  It keeps everything located in the stock small block positions.  It would be best to get all the accessory brackets and pulleys with the engine or you will end up searching everywhere to find them.  Or you can buy trick aftermarket pieces.   The existing small block altenator, A/C, and power steering pulleys usually work, except on '85 and up serpentine belt equipped G-bodies.  The power steering pump resevior must be changed to work with the big block brackets (or use the one already on the big block).  A custom bracket may be necessary to use the small radial type A/C compressor.  All the v-belt big blocks used the old long style compressor. All hoses and wiring should be a simple re-connection.   None of the hoses or wiring require relocation or modifying, so that is simple enough!

Power Steering Delete
If you would like to delete power steering, this is a simple bolt-on change.  Any manual steering box from 80's to early 90's vintage S-10 pickup will bolt-in directly with no changes.  You may or may not need to use the pitman arm that came on your factory steering box.  There may be other steering boxes that will fit, let me know if you know of anything else that will work.

Fuel Pump
You have plenty of room to run a mechanical pump if desired.  There are high volume mechanical pumps out there that will flow all the fuel you need for all big blocks, except for extreme cases.   I personally run a mechanical Carter Competition Series pump (part number M61045) that is rated at [email protected]  and requires no regulator.  If you have a street driven car, it is nice not to have the loud buzzing associated with an electric pump.  I ran 7.30's in the 1/8 mile and 11.60's in the 1/4 with no fuel problems!  See pic. 5 below: 

Pic. 5

Hood Clearance
You are going to be very limited to what intake you can run if you want to keep your stock hood.  You will more than likely have to use a low profile intake.  A few of the intake manifolds that will allow this is:  Edelbrock Torker, Edelbrock Performer (not Performer RPM), or Weiand action plus, Weiand X-celator.  Others will require an aftermarket hood or installation of a scoop.  You may be able to get away with an intake that is taller, but you will have to run a low profile filter.  The big block need mucho air, so you really don't want to restrict the air intake.  Right now, I have a 3 1/2" cowl hood, Edelbrock Victor Jr., 1/2" nitrous plate, and a 4" drop base K&N filter.  The breater touches the hood on the front edge! 

Front Suspension
The additional weight of the big block is going make the front end VERY "spongy" if you keep the stock front coil springs.  It looks cool, because it lowers the front end about 2".   Also, it is good for drag racing, because it allows the front end to raise up faster and get the weight transfer to the rearend on launch.  Unfortunately, it kills handling.  So, if you want it to be a driver, and handle well you will need to upgrade to stiffer springs.  Brakes can be upgraded, but I feel comfortable with mine, especially when I installed large rear brakes from a 70's LTD on the Ford 9" rearend.  I have never had braking problems.

Cooling System
A big block is going to generate a lot of heat, so your cooling system needs to be able to handle it.  You could install a 3 row factory radiator and maintain your factory fan and shroud, but I believe this would be marginal, and would have overheating problems.  Another option is to install a 4 row radiator.  Some '78-85 G-bodies came with diesel engines, these were equipped with 4 row radiators.  You could go to the parts store and get a factory style replacement.  I bought mine from a company called Wholesale Radiator. It is part number 858, cost $167, and they have them in stock. They have two phone numbers. Call whichever is closest to you. NJ: 1-800-631-5835 or TN: 1-800-345-3901.  Unfortunately, the end tanks are about 1" wider than the 2 or 3 row, so your factory shroud will not work unless heavily modified.  If you want to use a stock fan and shroud setup, you will need to find a diesel G-body in a junkyard and get the fan shroud and supports (if you can find one!)  I ended up going to the Chevy dealer to get the wider rubber "cushions" that the radiator rests on. I used brackets (that came with my electric fans) to secure the top of the radiator to the radiator support. I used electric fans with a built-in shroud (Flex-A-Lite 220 or 210). Works great and keeps the big block cool!  See pic. 6 below:

Pic. 6

Of course, a custom aluminum radiator would be really nice!  They are available, but are pretty expensive.   Be Cool makes a bolt-in, part number 60018 for manual transmission ($491.99 in JEGS) or part number 62018 for automatic transmission ($541.99).  Later, I ran a "standard GM style" Griffin 31"x19" 2 row aluminum radiator.  You will have to fabricate brackets to make it work.  You can get them for around $170.  Griffin and Howe also may have specific applications for G-bodies.

UPDATE:  I have been informed that Wholesale Radiator is no longer open for business.  Here is source information provided to me:   Radiator Express, the phone #  is 1-866-723-3977.  They have a radiator for a 1978 Malibu classic V8 5.7 liter, the radiator is a brand name and best of all it is a 4-row (26 X 17) for $182.95. You can also go to there website at Be sure specify that the radiator is for a Malibu classic.     

With 454's an external balance dual bolt pattern 168 tooth flexplate (GM part#-343738) is required.   This will also require a different starter.  I recommend a gear reduction "mini-starter" in order to ease installation and removal of the starter.   Otherwise the header may have to be loosened to remove the starter.  You may also experience hot-start problems even with the high-torque factory style starter.   Most mini-starters will work with either the 153 or 168 tooth flexplate.  A 396 or 427 may use the existing small block flexplate when installed in 1985 and earlier models.  Newer models require the earlier flexplate.  I also recommend installing a remote Ford style selenoid for your starter.  This way, you only have one wire (battery type cable) running to your starter.  This means you have less wires to worry about getting melted on the headers!  Also, you have a good place on the firewall to connect a starter "bump" switch to start the motor outside the car, or turn the engine over to adjust valves, etc.

Transmission and Crossmember Options
There are many options for transmissions.  You can use a TH350, TH400, 700R4, 2004R.  Of course, there are other options, but I am only going to cover these applications, since I don't want to get into a completely different subject of transmission swaps.  Any of these will work behind a big block, but depending on horsepower output, will need to be built to handle the additional power and torque of the big block (especially the overdrive trannies).   To get custom made crossmembers for ALL the applications listed above, check here:

First I will go over the TH350, since it is the most simple.  This is a direct bolt-in if your G-body did not come with the 2004R.  If your G-body came with the 2004R, and you want to use a TH350, you will need to get the non-2004R crossmember.  You will need to hook up a vacuum source to the modulator valve.  If your car came with a 2004R, it does not have this.

Next up is the TH400.  It is a big heavy transmission, that can take quite a bit of abuse with little modifications.  Unless you are really making over 450 HP and have your car hooking up, you might want to consider a TH350.  The TH400 is heavier and takes more power to drive it (less efficient). But, if you want solid as a rock performance it is the way to go.  That being said, here is what's required to fit it into your G-body.   For 82-up G-bodies, this is a straight bolt-in application if you use the 2004R crossmember.  The 78-81 G-bodies will require a frame extension on the drivers side in order to use the 2004R crossmember.   You will use the stock crossmember, transmission mount, cooler lines, shift linkage, dipstick tube (must be slightly bent), and driveshaft if your G-body was originally equipped with the 2004R.  You will have to use the yoke from the TH400 transmission.  Please see the other tech article on my page for more on the TH400 swap.

Next is the 2004R.  If your G-body didn't come with one, and you want overdrive, then the 2004R is a good choice.  It is a direct bolt-in on 82-up since the G-bodies came with it from the factory.  Also, the 2004R has more desirable gear ratios than the 7004R.  Also, this transmission will have to be heavily modified to handle over 450 HP.  Here is a chart of the difference in gear ratios between the two, notice the big drop between 1st and 2nd on the 700R4:

. 2004R 700R4
1st Gear 2.74 3.06
2nd 1.57 1.62
3rd 1.00 1.00
4th .67 .70

All you need to do is find a 2004R crossmember from a junkyard to bolt it right in (82-up models).  Earlier models will require the frame to be extended on the drivers side.  Try to get the other pieces that come attached to the tranny, such as the TV (kickdown) cable.  The 200R4 has gotten a good reputation as a strong transmission if built properly.  It cost much more than a comparable TH350 or TH400 build, though.  For more info, go to Art Carr's homepage. 

Last, but not least, is the 700R4.  Unfortunately, there isn't anything that will work with this transmission without modifying it!  The driveshaft will have to be shortened, the crossmember will have to be modified to work with it (or get one from, and you will need to get a throttle cable bracket to adjust the TV cable.  These transmissions can be destroyed in a matter of miles if the TV (kickdown) cable is not properly installed (same applies to the 2004R)!  Also, this transmission will have to be heavily modified to handle over 450 HP.

If you use a TH350, and the car came equipped with one, then there is no change!  If using a 2004R transmission, then get a driveshaft from a 2004R equipped G-body.  The TH400 will use the 2004R driveshaft, but you do have to use the yoke from the TH400 transmission, because the yoke is much larger than any of the other trannies.  The 700R4, it will have to be shortened. 

This is a definite weak point of G-bodies.  The factory rearend is the weakest rearend GM has produced.  It has a 7.5" ring gear and wimpy 26 spline axles.  I have talked to a couple of guys running this rearend behind a big block, but I think they would tell you that it is something they constantly worry about breaking.  I don't want to go into rearend swap here, especially since I have covered it in another article already.  Click here to see the tech article on rearend swap options.   I am just going to recommend that you upgrade your rearend.

OK, I hope I have covered about everthing.  If you think of something you feel that I left out, drop me a line with your suggestions.  Hope this helps answer some of the questions you may have had about installing a big block chevy engine into your G-body. 

Copyright 2000 Phillip Patterson. All Rights Reserved.