Have a 4l80 in my fd, fits fine. only thing that I had to clearance was making room around the electrical pigtail connector on the side of the trans. It fits without beating the chassis a bit but you can't unplug/plug it back in without adding some clearance. That being said its a big heavy bastard. I have a single 3.5" exhaust. may have been able to fit a 4" with a different trans crossmember.
I have a beefed up powerglide sitting in the garage that i sometimes consider throwing in, but for what I'm using the car for, for now I won't give up my lock up and being able to throw it in drive.
Glides are more economical and a hell of alot easier to rebuild/service.
with a turbo car you could run low gears to make up for the freeway rpm, even lower than 3.27 if you wanted. May give up some et but its will still move.