72 351C starts to overheat at idle

[1969_Mach1]

When does it overheat?  Usually, overheating at idle is insufficient air flow through the radiator.  Overheating at speeds above 30 mph is insufficient coolant flow.

What type of fan are you using on the motor?  After market flex fans don't work that well at all.

For thermostats, I've had best luck with Stewart brand high flow thermostats with the additional bleed holes.  I don't know if they make one for a Cleveland motor.  But I think you should have some type of high flow thermostat installed, probably 160 degree.

What is your base and total ignition timing set at?  If it's not advanced far enough the motor will run hotter than it should.

Is the motor stock, has it been rebuilt, is the cylinder bore stock or oversize?

I'm not too familiar with Cleveland motors, but isn't there some type of restrictor installed in the block down below the thermostat?  I know two people that have had that missing and severe overheating problems occurred.

Photo I found on the internet of a replacement of the factory installed restrictor.  Apparently somebody reproduces them.