My experience w/ 92 cx which also had the cat in the exhaust manifold: Swapped in b18c1 w/ JDM 4-1, cx piping went all the way to end of header so I hooked it up to drive w/ til my exhaust came so I wouldnt keep up the neighbors. Basically the exhaust manifold was the same length, the exhaust was just longer to take up the space where the cat would have been. Using SI exhaust now w/ custom cat. Just swap ex header and cat w/ dx hatch cat back. Shouldnt need cutsom piping.