I recently rebuilt the 350 in my 1979 K20. Flat top pistons .40 over, vortec heads, mild cam, Holley 670. Nothing too exotic. First time I started it and revved it up to 2000 to break in cam and it only went about 5 minutes before the water temp was pegged on the stock gauge. Radiator was leaking too. I shut it down. I replaced the radiator (3 core) and also put in a new aftermarket mechanical water temp gauge just in case. This time it only ran about 4 minutes before it hit 250 degrees and counting, so I shut it off again. Wondering what might be causing this? To fill with coolant, I just filled the radiator before starting, then topped it off after it had run a minute or so and put the cap on. Wondering if I missed something there and maybe have trapped air?