this is one of those times I would love to have an old engine analyzer. I think one of those would be key to figuring this out.
An ignition oscilloscope is a fabulous tool. They've gone out-of-fashion. About all you can get now is hand-held 'scopes with very limited functionality. God bless eBay and used equipment.
after the cap and rotor, and the engine dropped into closed loop mode, the LTFT turned to 128, the O2 was switching from rich to lean, and it did run better. Oh and we tried another computer from the same year and model truck, no changes at all.
You've made real progress.
At this point, it'd be worth doing a cylinder-balance test.
Warm-up engine, then shut engine off.
Connect a vacuum gauge to manifold vacuum.
Dull the sharp tips of eight SMALL nails on a stone or bench-grinder.
Wipe a smear of silicone dielectric grease on each of the nail points.
SLIDE the tips of the nails between the plug wire and the distributor cap boot so that the nail travels up near the metal terminal of the plug wire. DON'T PIERCE the insulation of the plug wire or the boot.
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Start engine.
Ground the lead of a test-light, or use a grounded jumper wire to touch each of the nails in turn. You'll short-out the spark to each cylinder, one at a time. Don't short the cylinder longer than 5 seconds at a time, and let the engine recover for 5 seconds minimum before shorting the next cylinder.
Watch the vacuum gauge as each cylinder is shorted. The vacuum should reduce the same amount for each cylinder. Any cylinder that DOESN'T reduce vacuum as much as others is weak. Any cylinder that doesn't reduce vacuum at all is dead. My truck had two dead cylinders at idle--#1 and #3. Turned out to be a failed intake gasket. Squeezed out of position, allowing a direct air leak into those runners.
If every-other cylinder drops less than the alternate cylinders, you've probably got an issue with one injector, or a vacuum leak into one side of a dual-plane intake manifold.
Don't forget to remove the nails after you're done testing. I forgot once, had spark shorting to the air cleaner housing when I drove the car next.
Might be worth re-checking the IAC and TPS, along with the gasket under the throttle body.
How's you coil? You can take a spray bottle and wet the coil with water (not the wires) while idling. If it stumbles, your coil is bad which can cause your ICM to go out too.
Excellent test for a cracked coil housing, or a "carbon track" in the plastic.
I also connect a spark-tester to verify coil output power. There's several kinds of spark testers, this is my favorite style. The spark tester needs to be calibrated for HEI ignitions.
https://www.amazon.com/Performance-Tool-W86553-Ignition-Tester/dp/B003WZXAWK/ref=sr_1_11?dchild=1&keywords=HEI+spark+tester&qid=1614483342&sr=8-11&th=1