I would definitely stay with the 4psi cap when using old original radiators. Those radiators were not designed to hold a lot of pressure to begin with. Add age and you are asking for trouble.
Definitely check and/or replace the thermostat.
Temperature variance across radiator was in the proper range.
Another cause of temperature issues is a leaking head gasket. If you get combustion gases in the coolant, it causes little bubbles in the coolant which in turn cause the water pump to cavitate and not flow coolant properly. Open the radiator cap, run the engine and look for small bubbles.
