[Sirrotica]

You alluded to new head gaskets, I'm curious why the head gaskets were changed earlier?

If there was compression gases getting into the cooling system it's possible that it wasn't head gaskets, and instead was a cracked cylinder head, and is still getting compression gases into the cooling system, causing it to still overheat.

A simple way to test for compression gases getting into the cooling system is to take an old water bottle and fill it most of the way up with water, submerge the end of the radiator overflow tube in the water, then observe to see as the engine idles and warms up if you get bubbles from the end of the overflow hose.

You'll never get it to run cool if the upper portion of the water jackets are being filled with gases as the engine is running.

Not saying that this is your problem, but it seems you've tried most of the common cures for Pontiac V8s to cure overheating. Just a suggestion for diagnosing a problem that I've run into in the past.