Are you sure your distributor is coming in at the right RPM, not hanging up? Curved properly?

Not saying that is what I think..

I have always had issues with over heating even with a puller fan. A couple of problems I had. Clearance with divider plates/water pump impeller, fan type and clearance to shroud. Excessive/inefficient sealing of shroud to radiator, Air pockets in block. I jacked the front end of my car 8-12" and allowed the air to find its way out of an open radiator.

2 core aluminum? stock dimensions (l/w)?