In my experience, a Pypes X pipe for stock manifolds doesn't fit headers unless you buy new legs and cut them up. The actual X parts are different lengths as well. Not sure about other brands. I'd be willing to bet that with a sawzall and a welder, you could make them fit.
Pcesetter makes a catted H-pipe for their LT's. Last time I checked with them though they were waiting to do a production run and didn't have any stocked. That was a few months ago. I think if you buy your headers from stage3 they have a drop down box when ordering the mid-pipe and it is an option. But I know for sure that Pacesetter makes one.
Pacesetter H pipe is super simple and you could quickly fabricate one yourself to match any system. Quality x-pipes are a little more difficult to fab and require you to be able to weld stainless to copy the better ones. I've heard it's hit or miss getting headers to match with a different brand mid-pipe. Although you could modify any system to work with the Pacesetter, including stock, as long as you can weld.
If you can't weld, or don't know someone, safest bet is to buy the pacesetter H pipe unless someone can verify something else fits up.