I have 2 questions about MUSTANG car parts?
What are you trying to gain by doing this. I am not beating you up for modding your 6 cylinder...that is your choice. Swapping driveshafts will not work because the 6 cylinder has a different transmission than the 8 cylinder. You would gain nothing by doign this though.
A GT500 Exhaust system would not work on your car because it is a 6 cylinder. It only has one exhaust pipe as opposed to the two that the GT500 has.
If you want an exhaust system for your car, go to an Exhaust shop and ask them to install one for you. It will cost about $500 and will sound decent as long as you use either Magnaflow or Flowmaster mufflers/