Overheating headache

I have a 71 demon with a 318 engine and three speed transmission. During a restoration the engine was rebuilt. Modifications to the engine included a performance cam, Holley 4bbl carburetor, electronic ignition, and headers. I had some initial issues with engine overheating especially when idling.
I replaced the radiator and put on a factory original fan shroud.
Monday I took the car for its first short outing in about 18 months. The engine ran at about 180 degrees when moving, but after about a thirty minute ride, while backing up my driveway and into the garage (about a five minute exercise) the engine temp quickly increased to past the 220 degree reading, and upon shutting down the engine the temp quickly rose to way past the 240 degree reading.
Antifreeze poured out of the overflow, but only after the engine was shut down.
This engine has less 200 miles on it.
Is it common for temp readings to go so high after shutting down the engine? I have no experience in this but know this cannot be good.
I am going through the cooling system now in an attempt to determine and correct the possible cause(s). I would like to blame the headers, but just don’t know.

Thanks guys.