FORD 460 Overheating Mystery

I wouldn't sell it yet before looking at several other things. In my previous post I asked if you can spin the fan clutch when cold by hand, check that. Also check your radiator at temp from inlet to outlet with your IR gun in a 4 in grid pattern and record the readings. Especially check in front of the fan clutch area on the radiator any cold readings will indicate a blockage in the radiator. If your cooling system is operating correctly you should see about a 50 F degree difference from inlet to outlet. Third thing to check is how far does your thermostat open in boiling water. I have seen new ones out of the box only open an 1/8 in, you should see a 1/4 in gap. What temperature thermostat do you have? Does your thermostat have a poppet in the flange? These 3 checks won't cost you anything and will tell you a lot about your issue.