Slant Six overheat at cruise speed, especially going uphill

could easily be ignition timing. what are you set at? make sure your advance is working properly. to check the mechanical, hook up your timing light normaly (vac advance disconected) then slowly rev the engine, you should be able to watch the timing mark move away from TDC. to check the vac advance remove the dizzy cap (engine off) and apply vacume with a pump, you should be able to see the plate move as you increase vacume.