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For the flathead to run efficiently, it needs thermostats for at least 160 degrees (stock), but preferably higher. I run 170; many recommend 180.
That said, I would leave the thermostats out until your basic cooling problem is resolved.
First, check the obvious
- Coolant in the radiator about half inch above the core.
- Hoses not soft, kinked, or collapsed.
- Correct fan installed and working
- Distributor timing adjustment advanced, certainly not retarded.
Next, investigate the coolant in the radiator.
- No more than 50/50 antifreeze; preferably less, like protection to zero degrees, not minus 30 degrees
- test for products-of-combustion in the coolant, indicating a head gasket leak
- Watch the water flow into the top tank is vigorous at idle.
If all of that checks out okay, suspect the radiator.
- Are all the fins straight, or a lot of them laid over.
- Drain down the coolant to just empty the top tank; do the tops of the coolant tubes look crudded over? Do some of the tubes not drain down? If either is true, you can try flushing the cooling system with a good flush product, but the radiator will probably need to go to an old-school radiator shop for cleaning ($$) and repair, or re-core ($$$).
Maybe report what you find.