Stop in for a cup of coffee

MAYDAY MAYDAY MAYDAY!!

Stuck alongside road in a 1943 M3 White with a flat head 6 Chrysler. Coolant pushing out from the cap. Will not go into the over flow tank, new hoses, new radiator, new thermostat, which has now been removed and still pushing fluid out. Overflow Line has had a brush ran down it, not clogged. Coolant is brand new. Engine temp is is steady between 140-155,
Sounds like you lost the head gasket and you are pushing compression into the coolant jacket. Otherwise, the coolant would simply expand to max volume at the temp and stop overflowing.

It’s also possible that your radiator cap is fubar.