0 psi was standard in 1937.
Is your cooling/heating system 1937 parts?
261s were used in trucks with 4-7 psi radiator caps, depending on the year.
Try the 261 with the no-pressure radiator cap.
If you get "boil over", you have serious problems. You should get expansion overflow until the coolant overflows enough to reach normal operating level in the radiator.
