If you already have a cat-back system you should go with an aftermarket H-pipe or X-pipe with Cats.
Other than that you might want to swap your headers out for some long tubes, but I think the best gain in power is by removing all 4 of those stock cats. I scrapped my cats and that made a HUGE difference. Now, I know that you can't scrap them, but I'd go with high flow cats atleast.
Plus the sound is awesome
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Driving: 1998 F-150
Far way in Edmonton==>
1995 Mustang GT