I'm pretty sure he is going to replace the cats somehow, that's what he said in his op. He wanted to know if he should go with stock or aftermarket cats.
I agree with lowflyn on the piping. The easiest way is to retain the 2.25" piping all the way through. A takeoff GT catback will retain the 2.25 piping, though they can be harder to come by these days, I'd check a local salvage yard for a GT or cobra. I don't know if exhaust shops fabricate 2.25" piping, I'd imagine so, but I haven't been to one in a few years