340 Overheat / Radiator Geyser

The fact it didn't geyser until you shut it off shows coolant was circulating... Depending on how the radiator inlet is baffled you may or may not see flow...
When you shut the engine off the coolant stopped flowing and the coolant in the block got heated to the point of turning to steam... Hence the geyser....

Two years ago I got a car back that I'd owned twenty years prior... The interim owner had complained about overheating since the 340 was rebuilt.... I'd hoping to drive it home from Texas but after driving around for a couple days it overheated so severely I thought I'd cracked a head or blown a head gasket.. Had it transported back to California & started running tests... Block, heads & gaskets are all fine... I tried lots of things... In the end the only thing that helped was adding more base timing... I have 34 degrees initial timing & I have the advance locked out... The engine runs right at 180 where the thermostat keeps it...

Try more timing & see if your 340 wants more....