71 350 running hot coolant backup up problem

[Gregski]

hello, I hope you can help me out with my 1971 GMC 1/2 ton basket case running hot, that I put together, it has a crate Mr. Goodwrench long block 350 SBC in it bought in the mid 90's but its not a roller engine its a flat tappet cam, with a 350 transmission

I don't know if it ran before I got the truck I don't know if it overheated before I got it either, the engine was out of the truck and I put it in, the transmission was already married to it, I have no idea why the engine was removed from the truck to begin with or if it came out of a different truck

the engine has an Edelbrock Performer intake on it and a 1407 Edelbrock 750 CFM carb with an HEI distributor with the vacuum hose connected to Ported vacuum, which pull 0 Hg at idle, the manifold vacuum pulls 21 Hg at idle, I am not sure what the timing is set at at the moment but the truck starts easily and idles good and smooth

I do not have a heater core in it, so there are no hoses going to the heater core, there is also no bypass rubber hose from the intake to the water pump, I mention this because I thought the stockish SBC heads flow coolant internally through the intake manifold and so you don't need the bypass hose like you do on the Vortec heads

I have a brand new 195 thermostat, currently I am only testing with water and not with 50/50 coolant mixture

outside temp today was in the high 58 F degrees, when I test drove the truck it was showing 215 F so I cut the test drive short

without the thermostat in the engine the water comes out the top rubber hose that runs from the thermostat to the driver side on the radiator, I disconnected it from the radiator so I could easily see if water was coming out and without the thermostat it does, this makes me believe the water pump is good and spinning in the right direction

I even back filed the engine with water by pouring it in that top radiator hose and down the thermostat housing and it took a bit of water, I was trying to eliminate an air bubble, which I thought I done did, water even started coming out the filler hole on the radiator so I thought mission accomplished, but no

with the new thermostat installed, the water topped off and the truck running, after a while it starts to spew water out the filler hole on the radiator, I leave the cap off to see what in the world is going on in there and thats when water comes out the filler hole, and my digital brand new temp gauge starts climbing 195, 205, 215, ... 225, etc so I shut her off and put my head in my lap, lol

sorry for all the words, but I am stumped and want to answer as many questions ahead of time as I can, as I know there are a lot of experienced wrenches on this forum and I value your feedback and need your help