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It's the coolant pump!
The Milodon pump, like the Weiand pump, is designed to operate without a thermostat. The recirc passage of the Milodon coolant pump is not drilled out. So until the thermostat opens up a bit, there is no coolant flow through the engine.
Not good for the engine.
You have two three choices
(1) remove the thermostat
(2) remove the pump & drill the passage open.
(3) while the pump is off, replace it with the Flow Kooler pump.
Also, the Stant 13468 thermostat is nothing like the original thermostat. The original thermostat, made by Robertshaw, has been sold by many companies, always identified by the number 333. I would strongly suggest replacing the Stant 13468 thermostat too.
Need two bits of info for the picture caption:
Your father's name
The chassis number
-G