This may be a dumb/stupid question: When initially filling a new engine, and start up, could you just leave the radiator cap off to allow any trapped air to escape once the coolant heated up enough and the Tstat opened and coolant started pumping through the radiator and hence, the whole system? Just wondering, cause this is what I did with my son's Mustang when the new engine was initially started. Took a while, but eventually, you could see the coolant flowing through the radiator with the cap off.

Dave