That would be about right. The temp sensor is slightly below the level of the top of the radiator level so some coolant would come out.
Try it without the thermostat. Usually the thermostat slows the water down as it enters the radiator so it has time to cool before going back into the engine. But it won't hurt to try it for a bit without it.
What brand t-stat are you running? It sounds like the t-stat is not opening properly as the engine heats. But you have replaced it once.
Could the water pump openings be partially blocked not allowing the water to flow properly? Could the impeller in the pump be bad?
Take the t-stat out. Fill the radiator to the top. Start the truck and watch for bubbles. Might have a few for a minute, but should stop. If you get bubbles while it's running and they don't stop, then you have compression leaking into the cooling system. May be the headgasket tore while installing the head, or the head may be cracked. They sell a dye you put in the water that tells if your have exhaust gasses entering the cooling system. Might see if your parts store can get you a small bottle.
