Good detective work Frank!
Is the distributor a Mallory Unilite? It should have come with a little tool for changing the advance curve. If your overheating is due to timing, you probably need more advance at higher RPM. I would try that first before changing springs.
I would suggest, though, that it's probably lack of cooling airflow at speed - your web site doesn't show a fan shroud - What about the fan itself? Original 4-blade fans don't move a lot of air. These are pretty easy items to fix.
Gotta love that Autobahn for high speed tuning runs!
