All of this can be done easier without the t-stat installed!
To tell if it IS your head gasket, start with the engine cold, fill the radiator, disconnect the overflow tube from the overflow tank, then crank the truck an let it run (and rev the engine hard a few times - not for high revs, just quick full open/close throttle bursts) for 45 seconds to 1 minute (not enough time to get the coolant hot). Watch what comes out of the overflow. If you only get small drips of water, then the coolant is expaning as it heats. If you get "geysers" of coolant coming out, especially when revving the engine, possibly followed by air, then your HG is blown. Immediately after you shut off the engine (BEFORE THE ENGINE GETS HOT) open the radiator cap. If you get pressure release, it is the HG.
Also, with the cap off, and the engine running, check to make sure you have coolant flow across the radiator. If not, check the radiator core and the water pump.
