What performance parts can i add to my 1996 v6 mustang to make it sound more like a gt?
Like others have said, there is ultimately no way to make your V6 sound just like a V8... without actually having a V8. However, there are ways to improve the sound of your 3.8 in areas people generally associate with V8's, such as increased volume, a more "throaty" tone, or an idle with more "lope."
If you simply wanted a lot more volume, you could simply add a high-performance exhaust system like a dual pipe Flowmaster system or the like. I advise against this, personally, because I find that virtually stock 3.8's w/ high-flow exhaust systems tend to sound very raspy at all but near-idle RPMs. Personally, I always recommend a GT "take-off" system, which is a factory exhaust setup from a GT custom added to your car. This is noticeably louder than stock without the tendency for annoyingly loud rasp.
Ultimately, however, one of the best ways to improve your exhaust tone would be to have a company like Super Six Motorsports or Delk Performance custom spec you a cam (and for power gains, consider adding heads at the same time). I have attached a link in the source section of a 3.8L Mustang with Heads, a custom cam, and a GT take-off exhaust system.