Another overheating situation 63 Slant six

We need some more info.

What car
what year
What modifications

Sounds like you have a circulation issue. With no t stat the coolant should be circulating no matter what.

The cfm of your fan is good, pushing is not as efficient as pulling.

There are a number of things to check.

The water pump pulley diameter to crank diameter, find out what oem is.

I'm not familiar with slant 6s but on v8s you can pull off the lower hose and see into the water pump. If you can, see if the impellers are there. With the fan belt off see if the impellers turn with the pulley, maybe even insert a dowel to prevent the impeller from turning and see if the water pump pulley can be turned.

I think it was 67dart273 that has suggested take the lower hose off the radiator and put the garden hose onto the filler. With the hose on, the water should run out as fast as you can put it on ( maybe not on full blast)

If you drive the car at 40mph does the temp go down?

If you can get the oem rad fixed you will be in better shape than all other option, it fits, you can use the mechanical fan, an oem shroud might be available.

Get the correct thermostat for the engine and year.

Get the correct rad cap.

50/50 coolant