I am putting my two cents out there even if my experience comes from a '89 with a 350. Some things that have not been mentioned yet are:
- Lambda. My TBI had a really old Lambda sensor. It would run great from cold until the engine warmed up enough for it to start using the lambda readings and adjust fueling based on that. After the temperature was at 1/3 on the gauge or so, it would start to run more sluggish and sometimes throw a CEL. A new lambda sorted this, even though the fueling doesn't seem as crispy in closed loop mode. The fact that your thermostat was broken or removed and the car wouldn't warm up fully, but still ran pretty well, might point into this direction too.
- Coil. My truck has 160000 miles from new and was on the original coil. Other parts like dizzy, ignition modules, plug wires had been replaced before but the coil had definitely sat in its mount since Detroit, 1989. I ordered a new part from Rockauto and installed it. The idle misfilre which would happen every 5 seconds or so was totally gone after that. The module I used was supposedly a high output unit, a Standard brand, called Blue Thunder, or Blue Oyster, or Blue Whatever, honestly I can't recall. Seems to have been a good investment anyway. I didn't have any definitive symptom which led me to replace the original coil, but ageing gets to everything. Experience from working on 80s motorcycles tells me it's easy to spend days hunting a fueling issue which is due to an aged coil which gets intermittent as it warms up, so I tend to replace those on the bikes as a matter of course if there's any running issues, and applied the knowledge to this context with success.
I hope you get your 454 cured from its present state. The investment for these parts is small, so consider them also.
Edit: Oh and as someone above said, check and clean all grounds, that's a really important starting point and can not be emphasized enough.