Coincidentally, my '60 NewYorker (w/496" rollercammed stroker engine) is has currently developed hot running issues during the highway trips, while staying fairly cool with slow driving.

Underhood temps are also very much an issue with this car, especially when both carburators are hanging over the exhaust headers, sucking in well-heated air.

I need to add more vacuum timing in this engine, but it's rather cumbersome getting it right. The engine is temperamental and the underhood heat isn't helping.

Might want to go digital on the ignition and do some airdam fabbing as well.
Also checked the lower radiator hose and it appears to have no spring inside, which is mostlikely the main issue for the higher highway temps.