Pull the thermostat and drop it into a pot of boiling water on the stove.[ while you're cooking dinner for the missus ] It could be faulty, I have had about 30% that were [ except factory GM ] [ costs nothing to check it, but fatal to your engine if you don't ]
What pressure is the radiator cap? ( for every 1lb rated on the cap, your water operating temperature goes up 3 degrees above boiling) and where is your water temp sender mounted?? If mounted in the front of the engine, it will fluctuate a lot with engine speed/water flow, as againts a more controlled reading if mounted in the rear of the motor..
I haven't read completely through all the posts either, but if you are running cool without the thermostat and hot with it, the problem seems obvious. We were having a little cooling issue with the 455 in my Sons rpu when it first went together and Mondello recommended getting rid of the thermostat and putting in resrictor plates instead, like Deuce Roadster pictured. We bought a set, except ours our metal and came with 3 different hole sizes. The one with the middle size hole seemed to be the best and dropped the temp to a very acceptable 180-190. It also cured another problem we were having because it let the system purge itself of all the air much faster. With the thermostat we couldn't get all the air out. With the restrictor in place we can fill the radiator completely and it took a couple more quarts of coolant than when we had the thermostat in there. Don
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