I had the exact same problem and spent a small fortune trying to fix it. At the end of the day it was the MP non adjustable advance in the distributor (sp?) that was causing my issue. I changed to a Mallory unit and solved my overheating problem instantly. The way it was explained to me was if the advance is not adjusted properly it will cause the engine to run hot. My explanantion is elementary but I hope you get the jist...good luck.