Something is wrong. I’m assuming you are using a good grade 50/50 mixture of antifreeze and water, and the cooling system is full and has been air bled.
You need to address this asap, or you’ll be quickly looking at another rebuild job. You should consider to stop driving this truck until you have this problem solved. You’ll have to eliminate these one by one: Dash gauge? Coolant Temp sensor? Radiator cap? Ignition timing? Valve timing? Thermostat [sometimes these are put in backward, or a new one is defective]? Radiator fan? Radiator plugged? Plugged up hoses?
If the temp gauge is showing “hot”, does turning on the heater and heater fan to max help?
When the engine is hot, the radiator fan should be spinning. And if you remove the cap [careful, very hot water may splash out!], you should be able to see the signs of water pouring into the top of the radiator. That means the thermostat is open, which is what it should be when the engine is hot.
Was there an overheating problem before the engine rebuild? If so, then the radiator would be a suspect. Wouldn’t hurt to put in a new radiator or take yours to a radiator shop for testing.
Since this is a rebuilt engine, I guess the next thing I’d do after the above is drain the cooling system and remove all the cooling hoses and eyeball them end to end, make sure they aren’t plugged up with something. Do the same where they connect to the engine and radiator. Sometimes during the rebuild, the ports are plugged up w/tape etc to prevent stuff from getting inside, but then they forget to remove it.
If that wasn’t it, I’d remove the water pump and make sure it was spinning freely. Sometimes during a rebuild, they don’t put all the spacers in correctly wen they replace the water pump. . This can cause the impeller to hang, or hit something, and you don’t get the proper water flow through the engine. Or it could simply be defective