i would not do it. unless you want a sleeper. the gt500 is 400lbs heavier than the mustang gt, 350lbs is due to the drivetrain.
so regardless of the body you put the drivetrain into, that car will become 350lbs heavier. how much are you planning to invest? to take the gt500 engine and change to an alluminum block will take $5k, to save 80lbs. thats not a bang for buck project either.
you really have 2 choices here.
1. take the mustang gt, forge the enigne and strap on a kb blower/intercooler.
2. gt500 with weight reduction.
a no option gt500 coupe with a 1/4 tank of gas should be about 3700lbs. rear seat delete, lightweight front racing seats and lightweight rims and that weight is now down to 3600lbs aprox. or less weight than a stock 03/04 mustang svt coupe.