If you refer to the Parts List or the Shop Manual, HJs (4 or 6) came with unpressurized caps and 4 PSI caps, The thermostats are 151 and 170 degrees. The difference is in the type of coolant used. If you used water or alcohol antifreeze, then you used an unpressurized cap and a 151 thermostat. If you used ethylene glycol, then you used the 4 PSI cap and the 170 degree thermostat. If your radiator is in good shape, you should be able to run a 7 PSI cap.