You can swap over the trany from the gtp into you're gt. If you swap over the motor from the gtp without the supercharger it will be slower due to the lower compression in the supercharged motors.
Best bet is to buy the car. And swap over EVERYTHING. Just swap over the transmission,Axles,Engine harness and the engine and supercharger.
You'll notice a real nice power increase and you'll have a new and working trans installed.