I knew when I typed it that I didn't ask the right question 
What I meant to ask is: where in the headers did the O2 sensor end up being installed? I have mine just downstream of the place where the collector starts - that is, just after the four individual pipes on each bank come together.
Sorry for the confusion.
Also, while I'm picking your brain, does the system require two gas lines, i.e., on from tank -> engine, and the other a return from engine -> tank, or just one gas line?
Thanks,
Tom