1970 1800E. Just had a complete engine rebuild, head and block were checked for cracks, none found. Block was decked, the head cleaned up (It's a later F head by the way) I can go 10-15 miles, the temp gauge is reading good, and confirmed with an IR gun. But soon, the temp starts rising rapidly, I pull over and I find that I am out of coolant!

The engine will hold 10 psi in the cooling system for 10 minutes. Testing for combustion gasses in the cooling system are negative. When shut off before getting too hot, you can hear water boiling in the engine, and the overflow tank gets quite full, well above the max mark. Oil remains clear, no sign of water, and no signs of oil in the cooling system. There are no visible leaks. The tube in the head is in place and positioned correctly. If I stop before it gets hot I still hear boiling. Then when it cools, the overflow tank is empty.

I'm totally stumped.
HELP!