You're having the same kind of customer service that I got from those guys. Good products, but you get a different story every time you call. My trans is prior to their "kit", I'm using a modified G force 2004r crossmember. However, my 81 required the same frame extension you're talking about for the later crossmember. It's a 1x3 piece of rectangular tubing that bolts to the original crossmember mounts, and is about 2 ft long. Also bolts to the frame a little further back, and the crossmember bolts to it. If you've got a hacksaw and a drill, you could make one of these from tubing.
Basically, I wouldn't be afraid to try the frame extension route. I'd definitely start there before cutting into the crossmember.
They may be overblowing the tunnel modification. I had to ding mine in 2 spots and cut the shifter hole. Unless they're using the stock 4 speed pedestal, but those are available all over. Although one of the dings I made might interfere with the flange on that pedestal, it's about the same area. Hmmm...good luck
You'll really like the dual friction, Got about 10K on mine and couldn't be happier. Smooth as butter with mechanical linkage.
The driveshaft thing really suprises me. I thought all G body was the same frame, except elcamino. You'd think if they can make a transmission crossmember that fits, they'd have a pretty good idea of where the tailshaft is gonna end up. But then again, they make custom speedometer cables for everything, instead of one or two mechanical adapters. I use an adapter from Forte's. Think their product line started with mopar stuff, so their technical staff has sort of a reverse threaded approach to start with
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