After trying everything under the sun, looking like those aluminum heads are just bad castings that have a design problem as they cannot cool properly.
It's been Process of Elimination to get you to this point so far and you still have the same over heat issue. Common denominator > same heads.
440 Source made an aftermarket aluminum big block water pump, passenger side inlet.
It overheated the engines they were installed on because the internal was cast too small and would not allow proper coolant flow.
All that Glitters is not Gold.
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