Hy redtruck65MA, You are correct when the coolant is cold the thermostat should be closed. The temperature for the thermostat could be anywhere from 160 degrees to 195 degrees [ I like 180 myself]. As for the pressure cap, if the engine is in a 65 truck the correct cap for that vehicle should work fine. The reason it overheated is probably because with the thermostat stuck open the coolant circulates too quickly through the system, and doesn't have time in the radiator to "radiate" its heat away. Hope that helps.
