Jon,
In 1942, all trucks were equipped with a 140 degree thermostat; and, a 166 degree thermostat was recommended (for better heat transfer) if you used permanent anti-freeze. Most likely, the 180 degree thermostat is not the cause of your "problem".
The coolant is not supposed to reach boiling temperature in these trucks, so there is no need to get a high-pressure cap to allow the temperature to raise above the normal boiling point. I think a 0 psi radiator cap was standard on 216s and early 235s. By the mid-50s, Chevy trucks used 4 psi caps and cars used 7 psi caps (with the later 235s and 261s, a 160 degree thermostat was standard and a 180/190 degree thermostat was recommended/allowed on large-truck engines).
Before you start trying to further diagnose your "boiling over" (most likely it is not boiling over, it is a pressure "blow off"), be sure you are not overfilling your radiator. Since there were (normally) no overflow tanks on these old trucks, and the coolant expands as it heats up, it is normal for the coolant to overflow if the radiator is overfilled (Does your truck's radiator have an overflow tube? Is it clear?). The normal level of fluid in the Chevy-6 radiator is about 1"-2" below the cap. Have you let your truck warm-up and cool-down a few times to see if the "blow-off" stops at a certain level?
Tim
