[JPBrecheisen]

The radiator tubes might be closed up or the fins are not attached to the tubes. I had flushed a hot running engine before also but when I had the radiator worked on half the tubes were still plugged with scale. Guy tried to rod it but couldn't get it right so had to re-core it.

You might also check timing. Late ignition timing can make a motor overheat because it cooks the exhaust valves and the coolant does what it's supposed to and takes the heat away, thus higher water temperature.

My guess is that it's more related to the radiator than the fan.

I live in a rural area and I've run several of them without a fan at all without issues going down the road. There is sufficient airflow at even relatively low speeds to keep them cool.

Double check the timing and make sure your damper hasn't slipped or is loose on the keyway. Saw that happen with a friends 250 I6 before.

Lastly I'll mention fuel mixture. Lean conditions.(vacuum leaks) will also make them run a little hot sometimes.

Just a few other things you might consider checking.