What was the indicated temperature on the dash gauge when the cooling system "boiled over"? Chances are there was an air bubble trapped in the cylinder head when you filled the cooling system, unless you drilled a small vent hole in the new thermostat, or "burped" the system by loosening the upper hose as you filled the radiator. Unless you could confirm a coolant temp of 200+ degrees in some way, it probably wasn't actually boiling. Expanding air can push coolant out of the overflow, especially if you have a "zero-pressure" system, or maybe a 4 pound radiator cap.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!