Ford Mustang V6? That car should already be making ALOT of noise, even without any modifications. Man That's a sexy car!!
Anyways going back to the question, well its a ford mustang, It just HAS to have a loud noise coming out of it, IT'S a angry, tough and wild car, and without loud exhaust noise, you're taking away one of the best feature of the car.
I've got a Zetec Fiesta, and I put a new exhaust back box and tip. And to be honest the exhaust sound is so loud my mom can hear me coming home from the other side of the city.
But remember, my car is a Zetec Fiesta(08 plate), theres a ££s gap between mine and your car.
For a mustang, i would recommend a big-bore exhaust trim.
have a look at this website, i bought alot of accessories they also got a BIG selection of different type of exhust pipes:
Hope that helped!