If you have been running rich since the magnaflow cat went on, the excess gas is what probably burnt it out. The new cat may not have fixed your high HC's, only masked them to get by the smog check. There is a good chance you are still running rich problem which is what led to the premature failure of your cat. How old are the O2 sensors? If they are the originals, or have more than 80k on them, it is time to change them. When OBDI sensors get old, they get lazy and tend to cause the engine to run rich, commonly seen in a loss in mpg's.

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