Stock V8 exhaust pipes are 2.5" I believe. Thats from the header to the tailpipes. You shouldn't have any problems with fit or flow going with aftermarket exhaust parts whether it's just the replacement of the stock H pipe or headers or the cat-back. For the best bang for the buck, replace the stock H pipe that has 3 cats per side and go with X pipes. The 99+ GT's have only 4 cats compared to our 6.
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