I'm listening to everyone. I am going to try a carb change before changing the heads but I'm skeptical that just because Auto Zone says I need an XY carb, is it really manufactured to 1969 specs? It could be a problem too.

The rebuilder did richen up the carb after running it on the test stand...by rejetting and new metering rods...don't know anymore than that about the carb other than it is a Carter 4711S.

Point is, the engine cooling performance is EXACTLY as it was before the extensive machine work, balance, blueprinting, and rebuild. The only removable engine parts that are the same (other than the block) are the heads, carb, and water pump housing and I've already verified the performance of the water pump and housing on the engine stand. That leaves the Carb, Heads, and Block.