[JayBar]

Jim,
When I had my 351 in my car I never had overheating problems.
Since installing the 408 last week, I noticed it running hotter.
I got stuck at a bridge opening the other day, air temp temp about 92F. degrees. My engine temp rose to about 100c then I guess the fans cut in and it slowly went down to 90c. And stayed there for the rest of the stop. A few minutes later I was crusing at 60mph and noticed the temp went up to 95c.
I am running a "restrictor" instead of a thermostat. My timing is set at 30degrees of advance. My plugs look like I may be running a bit rich, but then again I haven't been able to pull a plug after a steady speed cruse. I have to slow down to 15 mph to get home to pull the plug, so that may make the plugs show "richer" than they should be.
Your problem has to be with the engine not the radiator.