I didnt want it too loud so I decided to go with Corsa http://www.corsaperf.com/mustang.htm, I can enjoy very loud sound for a short while but if you take long trips you can get sick of it.
Now if you want sexy sound try this
http://video.google.com/videoplay?do...=05+mustang+gt

 

Not sure what a classic Mustang sounds like, since I was about 10 years old when they were out there, I agree with the poster that said that you don't have to do anything to a GT to make it sound that way. First time I drove my GT to the beach, my dad asked me if I did anything to it because he said it sounded pretty loud, and I told him I didn't, that's just the way they are supposed to sound. He looked at me like I was crazy and I just smiled.