Hello, my car is throwing code P0420. I believe either the Cat is bad or the cat could be gutted. If I put an OBX header on, can I put a High flow cat to it and then put a cat back exhaust on without any welding? I am looking to put a hi flow cat on just because I rather be legal. Is there an aftermarket header that a hi flow cat will bolt up too? Also what are some of the best hi flow cats for a 2003 ZX3?