Joey, From 1999 to the current year, it is not recommended that you use a pacesetter adaptor/h-pipe. They can be used, but you will have to modify the pacesetter so much that you might as well have a custom H-pipe made for your car.
You don't need new headers. You can buy a cat-back system made for a GT of the same year and then take the system to a good muffler shop and have them custom make an H-pipe and fit the cat-back exhaust. We are talking a max of $300 to have this done. I have heard usually around $150.
Try to stick with a system that uses 2.25 inch diameter dual tubes. This is the factory size for most GT's and you can sometimes find a used GT system for a decent price. Just make sure it is not all hacked up. Saleen sells some "take off" systems that are practically brand new for a cheap price. 2.5" inch diameter will work ok, but unless you plan to turbo or supercharge, you will lose some bottom end power.
I hope this helps! If you got anymore question, don't hesitate to ask. The V6 Forum Rules

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-1998 Black Cobra, Kenne Bell 2.1L Supercharged
-2005 Mustang GT, Premium, 5spd, Redfire, Mac Catback
-2007 Roush Nitemare Stage 3 Supercharged #48 of 100