Yeah, the stock O2 sensor is positioned just before the catalytic converter, in the common area of the 'wye' where the left/right pipes join together.

BTW, did you have any fitment problems using the 3.4 exhaust manifolds?

I understood from seeing posts at other forums that there might be an advantage to using these, because they have larger (2") outlets than the 2.8 manifolds.

Did you have 2" downpipes fabbed up back to a 'wye' into the cat, or do it differently.

Also how does it sound and perform.

'90 Troop 3.4 LS
'89 Troop RS (Has Valve Issues, needs Counseling)
HI, I'm Ed and I'm a Trooper-holic!
Keep On Troopin'......