Well, not to mention, if you don't care about passing inspections (I assume you're over the federal 50k limit to get rid of the cats?) If you're going with this setup for a purely track car, then I assume loudness wouldn't really matter. Idk, I'd just go with a high flow cat to be in compliance with laws and inspections. You won't see noticable gains between a high flow cat and a a straight track pipe. If you'll be running a stock exhaust, without cats, that'd mean some type of fabrication would be required. You'd need some type of simple custom job made to connect from the headers to the stock cat-back system.
May I ask where you are going with this? And why use the stock exhaust?