You might consider designing the x-over pipe to go between the tranny and the front driveline, this way you could keep the O2 sensor in the stock place, and out of the way. I modified the Downey kit to run it this way, I didn't like how they have it designed to go below the driveline.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed