340 Overheat / Radiator Geyser

Did you buy a new water pump?
are the outlets in the right place?

i.e do you have a 318 340 one or do you have a magnum style one which is designed to turn the other way due to the different belt route on those later motors.
if its running the wrong way it won't work, but your symptoms would fit with hot radiator but little movement of coolant until switch off, and it boils in the head....

if it is the same pump it always had...this is a moot point

explosions/geysers

switch off coolant boils faulty stat pops open
or
switch off coolant boils and well it all comes out

Dave