The plug is just one of a few ports into the coolant jacket. I do not know if there is a true block drain plug let alone where it is if there is one.

 

T make this work the heater inlet has has to be lower than the engine outlet so as Kats says the cooler water can "drop" in. Some how the loop in the outlet hose has to go. Went out and looked at my truck, thought that what you had should work then noticed that the engine sits rear down, if inlet of heater is raised as compared to block outlet, rear of engine goes down, so top of heater may be to high as compared to the top of the engine, hot water does not like to go up then down thats why the hose wasn't hot last try. This has turned into quite an ordeal hang in there Laurie