Dr.Tile,When you say you're using the original radiator are you referring to the 1940 one or the one that came with the 250? If it's the 1940
radiator maybe it's cooling capacity isn't sufficient with the 190 temp thermostat after it opens. Also, how about the cooling fan? Is there space for enough air flow between the engine and radiator? If it ran well without a thermostat why bother now unless you'll need quicker heat from your newly installed heater in the winter.
The worse scenario is that the block is partially plugged from rust and the thermostat
worsens the problem.
