Do you plan to drive the truck in the winter when you'll need to use the heater? If so, use a 180 degree thermostat. 160 or below will not make enough heat to warm up the interior. If it's a summertime-only cruiser, use a 160 for 20 degrees more safety margin on the coolant temp. If you've got an original 1950 radiator, I'd reccomend a 4# cap max, as it's probably got one of the "honeycomb" design radiators that won't take the pressure a later type tubular radiator will handle.
Panic is right- - - -a thermostat only sets the minimum operating temperature. The size and condition of the radiator determines how much cooling gets done once the stat opens up and allows the coolant to circulate freely. A good-condition, correct size radiator should keep your coolant temperature below 200 degrees regardless of outside temperature or load on the engine, as long as the water pump and fan are operating correctly. A 4# cap will raise the boiling point 12 degrees, to 224 at sea level, less as the altitude increases.
Jerry
