I am looking at duplicating the 3" cat back piping that I am making next week. It would have to be welded to muffler, and I would put a flange on it for attachment to the cat. It will have no crush bends because it will be multiple pieces welded together for curves. Welds will be grinded down, and the whole thing will be treated with VHT paint black.
If someone is interested let me know…
Edit:
Also, if you want the full thing, 3" from the dp, high flow cat, and all the way back, that is possible too, but will obviously cost more b/c of the cat price and flanges etc…