If it has to have an O2 and doesn't the ecu could be in a full throttle or constant warm up mode, and causing it to run rich at idle.

The loss in fuel economy is something else though. I think the carb must just be dumping fuel into the engine to only be getting 14mpg.

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