MRT, MAC, BBK, etc. Most of them make an H-pipe for your application. You need the same H-pipe that every other 99-04 mustang GT uses, just get the right one for your tranny type.
Why not get a modular X pipe, like Basanni or SLP, where you can rmove the CATs when you want and add them back when you need to? I wish someone would make an H-pipe that did that!