Yes, you can do that. If you have a DX coupe or sedan, you can get an EX header, cat, and exhaust to be 100% bolt-on, but yeah, you can get an EX header, whatever cat, and have your piping shortened to bolt up the cat. You should have 2 O2's, and if you run an EX header, your primary will go in there, and the secondary will go in the cat(exiting end). If you get a universal cat to have put on by a muffler shop, you can get one with the O2 bung already welded in.