I don't think it would be all that difficult. Motor mounts would be the toughest part of the whole deal.
Get the motor as far back as you possibly can, that would surely put quite a bit of weight on the rear of the car.
Theres a guy named Dennis Butt that has a blue GT with a 3.8 that runs awesome. 10's if I remember correctly. He said it wasn't too bad except he couldn't fit an exhaust system on it so it has a fender dump. I've seen it a couple times and it looks like it really works well. Also shows those rustang guys how to properly run fast!
I also saw the GNT Mustang in an old GM Hightech mag from last year, The guy said it wasn't that bad of a job, Remeber you'll need the 200r4 out of a GN aswell !