These questions have been answered many times,and there are different approaches to the fixes. You MUST run with at least the two PRE CAT sensors installed in the proper exhaust banks or you will be taking risks ranging from poor engine performance and economy up to total engine failure from overly lean or rich conditions. Also,cutting holes in a 10,000$ car to fit 99$ headers is really as dumb as it sounds. The proper repair it to cut off and relocate the o2 bungs to a proper location. As far as sims go for the post cat, I reccomend you search those options when you get to it as there are many available fixes.
sent from my ATARI sc1224