While you have the radiator out for repair pull the thermostat out and place in a pot with water. Heat the water to see if the thermostat is opening. Preferably place a thermometer in the water to see what temp it opens at. Also, drill a 1/8" hole on the flat surface next to the thermostat valve. This allows easier filling and air bubbles to pass thru from the block to the radiator. I do this with all my thermostats.
Ron