Re: True Dual
if you get them headers they come with a y-pipe. It will have the o2 bungs (hehehe) already in it, 1 in each part of the Y by the header collector. That will take care of the uppers. Then, if you run no cats and straight pipe to a muffler, you will have to weld in a "weld-in-bung" for your 02 that is down by the cats. This is how I ran mine and it worked out great. Also, you should be given the option for NON-EGR or EGR, you'll want to get the EGR header so you can hook up the egr valve. What I did was cut off the front of the cat where its ovaled and ovaled a 3" pipe and welded it up, this way it was bolt on at the Y pipe.
I might add, I would NOT recomend running true duals, its very tight down there and you'd have to run like 1.5" pipes to clear, that small pipe will make it sound ricey and poppy. I think its better to run like a 3" as I did to a single in dual out muffler. And run the out 2.5" with 3.5" tips.