I have a 1976 ford custom with a 360 FE. Ive been have problems with it overheating for about 2 weeks. Its never done it before, since I had it, or my grandpa who bought it new. About three months ago I have replaced the belts, the hoses, the thermostat when I gave it a tune up. Also having the heater on high doesn’t help like on other trucks I have.
I would try removing the thermostat as a test to see if that solves the overheating problem. It’s not impossible to install a defective new thermostat.
Tester
Try a new radiator cap.
Assuming all the routine maintenance is up to date, and that flushed and filled w/a 50/50 mix for the coolant when you did the thermostat. The fact that your heater isn’t working suggests it is a coolant flow problem. Possibly an obstruction, or the water pump. Look carefully at the water pump. Is there any visible sign or audible noise that the drive belt is slipping? Is the belt tensioned properly? Either too tight or too loose can cause this. Next, there’s a little hole in the wp casing. Do you see any coolant, even just a minor amount, dripping from that hole? If so, the water pump is bad and needs replacing. The problem isnt’ the leak so much, at least at this point, but that things are corroding and breaking inside the pump and not pumping the water in as much volume it should. Also remove the hoses you put on, make sure you didn’t forget to remove labelling etc that could now be blocking the coolant paths. Also check the ignition timing again. Too retarded or too advanced can cause overheating. Best of luck.