Already time for a cooling system rebuild.

The rule of thumb is, if the radiator is old, it might be dirtier than you think. Usually shows up by overheating at highway speeds. The thermostats that we call "high flow" now, were equal to factory Mopar original thermostats up to about 1967. In other words, most of our motors are seeing a flow restriction with a regular 'stat that has the 1 1/8" opening. Spring for the EMP/Stewart thermostat.

--- And remove the block plugs at the bottom of the water jacket on each side, when you drain each flush. They're 1/4" pipe thread. 8 point sockets make it easy if they're the square kind. Use distilled water and coolant for the final fill