sounds like you done everything right, I would definately say it has a vapor lock due to the head swap. I seem to have the best luck with squeezing the heck out of the upper and lower radiator hoses like your pumping them up or something. this should be done with the cap off and the engine running. Youmay have already done this but if not its worth a shot. When i purchased my first ls1, the thermostat was froze....I had to go purchase a new one from gm. Im sure that isnt your problem but thought i would add that i have saw them freeze.