Welcome to the board!!!
From what I've heard, Borla is the way to go if you have the $$ to spend. You might have to get new side skirts with an opening for the exhaust. As far as companies that make a side exhaust for 99+ GT's, I think Borla, Magnaflow, and Classic Concept Designs all make a kit.
As far as sound, I'd go with Borla. A little more expensive but well worth the money. Hope I helped a little bit (I tried)
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91 GT AOD: 14.71@91.78mph
SOLD!!
01 GT 5-speed on order: Ordered: 06/13/01
Build Date: 08/20/01