You tried almost everything from retard spark, get advance more, get the mix richer, leaner, walk the dog, kiss the mother in law, etc... it always overheating quickly.
The cure is maybe this (pictures)
Earlier, many people think it was a good thing to save coolant when repair something needing to remove it, and, when the old coolant drop in the pan, (most of the time drop beside...) the coolant bring with him dirt, and dirt did goes back with coolant in the engine. the 2 rear cylinders in a inline engine are always the worst, on V8 it's mostly the same. It will need a good clean up. And you'll be able to avoid to kiss again the mother in law, since the overheating problem will disappear.
A good proper hot tanking of the engine block would remove any and all crud that is in the water jacket passages, as well as all oil galleries. Followed up by a good power washing.
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1957 Pontiac Pathfinder Deluxe sedan restored 261 six
1974 Chevrolet Caprice Estate wagon low mileage original 400 V-8
1956 Pontiac Pathfinder 2dr sedan, 496 - dyno'd 545 hp, stick shift, 4.11 posi - Hot Rod