Could be a number of things,plugged radiator inside or out could be plugged with dust and debris between the fins,can blow it out with compressed air and see if that helps. Could be the fan belt is loose not spinning the fan good enough at speed. Could be a stuck closed thermostat not letting coolant to the radiator,or it could be a bad water pump. Could be a bad radiator cap also,doubt it but is possible.
Can you open the radiator cap and start it and see if there is flow thru the cores while its running? Let it run for a bit and see if you can tell. Blow the Radiator out with compressed air from both the engine side of the radiator and thru the counterweight opening side. What is the concentration of water to antifreeze? is it 50/50?
Push on the fan belt at its longest free running lenght it shouldnt deflect more than a 1/4 inch at most. Does the truck have a fan clutch on it like a car does? Possible a bad fan clutch if it has one?