My 67 273 would do that after a coolant change. Get up to temp all ok. Turn off engine and you could hear boiling and if the rad was full it would dump a bit of fluid. I found that filling the rad with the upper rad hose off till the fluid came out the upper hose, fixed the problem. I also do not run an overflow so the coolant level in the rad is about 1 to 2 inches below the rad cap (like FSM says)
