Why don't you just get a replacement hi-flow cat for the YJ with CARB and OE numbers?? Anyway, to answer your questions. You might have to put a to make a O2 sensor hole somewhere near the header collector. If there is a tube going into the cat, I would flatten it out and weld it shut. Beyond that, I don't see what else would have to be done. Oh yeah, disconnect the battery if you have to relocate the O2 sensor! Also, if you do, make sure it is VERY close to the collector. O2 Sensors work best hot......