The GT has a stock H pipe on it. The V6 does not require any kind of midpipe due to the firing order. There is no high end HP / low end torque to be gained from using either an X or H pipe. You can run the pipes straight back from the cats to the exhaust if you want.
Here's the thing though.. a GT H-Pipe is going to be part of the cat system. therefore it would have to be cut and welded anyway, because those cat's aren't going to hook to the manifolds in the exact same way.
I had the shop fabricate me an X pipe, only because an X will be louder than a H pipe, or nothing at all. I have MAC Straight Flowpath mufflers, left the cats on (I think it sounds too ricey with the cats removed on the 99-04's.)