The o2 sensor senses percentages, not quantities. So it isn't looking for 6 cyl's worth of o2, it is looking for x% o2 in the exhaust. Several aftermarket headers (including one of the 2 for the Isuzu 4cyl) have the o2 sensor on one port, whereas the stock exhaust because it combined early and there was a convienent place for it after they combine.

I have a picture of one such header at: http://tad.grosvenor.org/gallery/display.cgi/72/DSC00007.jpg

I'd say if a dual exhaust is important to you, the o2 sensor shouldn't be a big concern. Just don't move it so far away that it isn't heated up properly.

-Tad

1994 Trooper. 3.2 DOHC/auto. 4.77s,3xARB, OME 919HD springs. 34" Swamper LTBs
2001 Trooper "stock" <img src="/forums/images/graemlins/wink.gif" alt="" />
http://www.planetisuzoo.com