You should be able to drive that set up 90 mph without overheating. Something is wrong with either, your cooling system (radiator, thermostat, crud in water jacket, low coolant level, fan clutch), or your ignition timing is off.
Carl hit the nail on the head! Melvin (my 70 GMC C2500) is 350 q-jet. TH350, 4.10:1 axle. Overheating never an issue, even (over)loaded with a large cord of firewood.
