First off, I know its a slippery slope, below is the knowledge I have gathered over the years ( theres not much LOL ).
There have been rounds and rounds and rounds of bench talk about this since Drive Clean came in to effect in 99. Everybody seems to have their own opinion about what they are comfortable with and how "legal" they want to be ( or think they are ). Ultimately its up to the Officer and if you disagree with the officers opinion then ask the Judge as to what you can and cannot do. FYI the last couple years MTO has been at the occasional car show at 400 & 7 , 427 & Queensway, and Californias on the QEW randomly pulling over a very small number of vehicles for inspection as people leave the show. They have been known to walk thru the car show as well and boy a lot of hoods are closed fast !
Per below the Gbody falls under 2 categories for emissions exemption from Driver Clean ( pre-88 and/or Hot Rod ). MTO Link below
Pre-88
This means that provincial officers — anywhere in Ontario — are authorized to stop and inspect your vehicle if they believe:
• your vehicle is emitting excessive exhaust smoke
• emission control equipment has been tampered with or removed
Hot-Rod
** if you can prove your vehicle had a motor swap pre-99 then you are exempt but the vehicle must meet or exceed 1980 emissions levels
- Otherwise
• Vehicle cannot legally operate if there are visible emissions for more than 15 seconds in any 5-minute period
• For Drive Clean testing, the vehicle model year is the model year as designated by the manufacturer
• Vehicle must meet or exceed what were the emissions standards for the original motor when all original emission control equipment was functioning
• All emissions control equipment must be attached and functioning (equipment refers to what came with the replacement motor or be equivalent to what would have come with that motor)
So if you can prove the motor in the car currently was orig equipped with dual exhaust and dual cats then you will be OK, otherwise it could be considered tampering. Everything else is up to the discretion and knowledge of the officer ( which is typically limited to a visual inspection of the EGR, Charcoal Canister & Cat Convertor ) then they may send you off to be tested.
So in my case I got the used TES Headers for cheap, plus a used / gutted hi-flow cat and then started saving for my cat-back. The exhaust in my car is Quasi-legal but able to pass the "visual" inspection for a regular V8 Monte Carlo. Under the hood I have the stock air cleaner, EGR, PCV, and Charcoal Canister all "hooked up" to my stock looking "305". ( you can figure for yourself what is in-between the " " )
I know a guy who is really struggling with this now as he's building a Twin Turbo LQ9 in a Gbody and he's trying to figure out exhaust options ( he used to run the 3" single exhaust on his GN ) as he wants to drive it daily in the summer and doesn't want the hassle of the MTO flagging his car. His friend ( my trans builder ) had his SBC w/blower Powered '39 Furd run thru MTO last summer and he ended up with a bunch of fines ( court case pending ).
Can you say Cash Grab ??