Like GOLDEN ARM said..

There are so many things to look for. Chrysler did their best to engineer everything to work together. If you've changed one thing that can upset the balance.

You should have the correct:

water pump pulley diameter
crank pulley diameter
T-stat or one slightly cooler.. 180. I like 160s
clear radiator, head and block passages
good water pump. I like the 6 vanes pumps
proper ignition timing
spark plugs
carb jetting/settings
heat riser working properly(open all the way when hot)

The newer head gaskets have tiny(slotted) cooling holes versus the original gaskets were round. I drill the head gasket holes to 1/2" for more flow.