I once had an overheating problem on a 75 or 76 Dart 318 ex cop car. At cruise the rad couldn't flow fast enough, so it would fill up the overflow reservoir until it started pushing coolant out its overflow. Eventually it would run low on coolant and overheat. I was in Hemet, CA then, and had a shop there rod-out the radiator. Man, that car ran so cool after that, no matter how hot it was outside.
If you can find someone who can still do that work, I'd strongly suggest that.
