Check your radiator cap or just replace it with a new 13lbs cap.
Don't overlook a bad water pump....
If the truck is getting so hot, that the over flow starts boiling over. Chances are that the thermostadt is opening, since the hot water is circulating from the hot motor. Check for the bubbles in the radiator, (for bad head gaskets).
Also, make sure your motor is timed correctly and that your lubrication system is up to snuff. Make sure you have oil in the motor
I have seen overheating motors due to lack of motor oil.
Finally, make sure your carb is tuned correctly. Make sure its not too lean.
Good luck, I'm still chasing my overheating gremlins, but I'm making it an experiment. I'm going to change items one at a time, just to see what works or improves the cooling system. Going to change Fan, water pump, radiator, ect...