If it fails with the new O2, pull the cat and have them check the pre-cat levels at idle.

If the new O2 sensor is a universal one, keep the wires the same length as the originals, longer/shorter wires can cause issues with the ecu receiving the signal from the sensor.

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