Curt, the head gaskets can easily be reversed which blocks some cooling holes, but this usually only causes local overheating on one cylinder. A few things to try:

Could the thermostat be reversed in the housing?

Flex fan needs spacer to set correct position, so that may be missing?

I wonder if its an ignition or valve timing problem since the dizzy was changed?

New intake manifold may be sucking air causing weak mixture?

New piston is oversize or rings too tight?