Is it a factory weld over the O2 bung, or is it a private job. If factory does Canada emissions require an O2 on your carb'd '90?

Since it is carb'd, there is an air injection valve(at least on the older ones), that adds air to the exhaust gasses to help burn the unburnt fuel. If this is bad you will also get higher HC's than normal, which a new cat will not fix.

Timing can effect fuel burn and cause higher HC's, though I am not sure how much.

Did it fail at both idle and 2500 or dyno tests?

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