did you flush the coolant system when you replaced the water pump? you need to any time you change the pump. You might have jarred loose some old crud, cause of the new Pump, and its now clogging something. First thing I would do is get the truck up to operating temp, check your hoses by squeezing them and feel for coolant running through them. Im assuming you removed the T Stat completely? kinda what it sounded like, I would definetly get a mid temp one and put it back in. Another thing that can cause overheating is a blown Head Gasket, do you have any inter mixing of fluids? Oil in water or water in the oil?