radiator overflow sends out coolant after engine shutoff

[Ironhorse]

The coolant "puking" out the radiator overflow after engine shutdown is completely normal for trucks and cars from that era and before. It does not do that during engine operation since the coolant gathering heat from the engine combustion is cooled while passing thru the radiator via the water pump and then returning to the heated engine. When engine shutdown occurs, the heated coolant no longer circulates from the radiator via the water pump and expands due to the latent heat. No where else to go other than out the overflow tube at the radiator opening.

That was prior to "Uncle" forming the EPA and a boat load of automotive laws and restrictions of manufacture. All the newer cars and trucks capture the coolant in a recovery type closed system which you can easily adapt to your truck. I did it on my '71 4x4 and plan to add the same type system to my '71 Chevelle. Not stock for the year of mfg. but keeps the floor dry, no sweet liquid for the family dog or cat to lap up and also keeps the radiator full with no air space. No air space equals more coolant in the radiator and the overflow which gives a larger amount of coolant to work with.

Your kitchen pot boiling the water analogy may or may not be correct. When you boil water with no lid it takes longer and the heated water emits water vapor which you won't normally watch for. If you were to place a lid on the pot and continue boiling at the boiling point the raised pressure of the heated water will escape thru the pot lid and make a mess on the stove. The radiator does the same unless using a recovery type system.