Reason I ask is, the '86 V6 Firebird i've been working on has the o2 sensor in the y-pipe, downstream of where the pass. side collector pipe comes into it. Is this where it belongs on this car? Conventional wisdom tells me that it should be closer to the cylinders, like in the manifold, but admittedly i'm not very familar with the V6's.

 

on Y pipe in front of catalatic converter. pop your head under the passanger side right behind wheel and you'll see it on the left hand side of the tube.

 

Ok, I guess i'm really not familar with the V6 setup!! That's where it is, behind the pass front wheel so I guess it's where it belongs. Thanks.

 

Yes on your ride.
On the 1995 3.4's I've swapped in, I have one per exhaust manifold, exactly where you suspected they would be.
My 1985 is exactly the same as yours.

 

Yep, that's the correct spot. I think it was placed there to get an accurate reading from both exhaust manifolds. (Remember, the 2.8 V6 was put in F-bodies for the gas mileage/insurance aspect!)