With a set of Kooks long tube headers installed, you'll need to choose an X of H pipe to pair with it. Kooks has been a leader in header and exhaust design for a long time. Their fit, quality of materials, and performance are hard to beat.
Although there is some debate on what works better, RRM likes the X pipe design better as we have seen the gains achieved buy these systems on multiple track cars. Kooks also provides two catalytic converter types, Kooks High Performance Green Cats or their High Flow Race Cats.