Intermittent engine performance

VIN Z TBI.
Five speed transmission 12'' 10 bolt rear end.

The mileage is accurate to frame, the engine was replaced before I bought the truck. Possibly rebuilt and was told it rumored to have 350 guts in it. Owned for three or four years now. When I first got it it had blown head gaskets. Replaced them, plugs and wires, and it ran fine, no rattles, no tics, no smoke, smooth. Then it starting having overheating issues, got close to red twice, replaced bad water pump with one off a 350 v8, swapped radiator to the big ones attached to the 350 v8 and put in a 185° thermostat. Performance wires, new distributor, new ICM. Replaced all those because it ate ICMs, new distributor fixed that.

Truck is not a hot rod by any means but it definitely surprises you, I fooled a lot thinking it had a v8 but over time it has lost power, oil pressure at first start is little under 60, after it has warmed up at idle pressure is 30 or lower sometimes hitting a low of 10. 50 during a decent acceleration. It does smoke some have to really look to see, on occasion it will create a small cloud after idling at stop light then accelerating, rarely will it create a small cloud on start up. It absolutely does not like it when I turn the AC pump on.

I will admit oil and filter has not been changed completely in awhile, what I mean by that is when I did my clutch I replaced rear main, oil and filter (year and half ago) but it still leaks so it gets a quart once a week. I did however remove the oil cooler lines, could never get them to stop leaking! Maybe that was a bad idea. This bad performance does not happen every day, but every other day which I do not really understand.

Is this engine toast?
Should I try some things before a rebuild/replace?

Anything is appreciated.
Thanks.