There is always the Force 2 Exhaust system from Flowmaster. I haven't done it yet myself but I am leaning that way. If you want loud you might want to think about the American Thunder package. If you get a convertible I would definately suggest Subframes and a Strut Tower Brace to stiffen everyting up. I plan on getting the Kenny Brown Extreme Sub Frame set myself.
http://www.flowmastermuffler.com/ http://www.kennybrown.com/
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2000 Mustang GT Convertible
Spring Feature Special Edition
Black/Black/Charcoal
Bone Stock