400ci/400THD pushes coolant out through overflow after engine shutdown. Happens even if system is not full. Engine is not overheating. 

I assume your overflow is a hose that dumps on the ground. I have installed an inexpensive coolant recovery catch can on my '72 Lemans with a 455/4-core radiator. A 3" x 10" is what I have. I bolted it to the radiator support. I used one existing hole and drilled a second. I mounted it literally right next to the radiator and connected the overflow hose to the input line of the recovery can. No more leaking antifreeze. When the radiator cools down, vacuum will draw the coolant back into the radiator.

 

https://www.speedwaymotors.com/Search?query=coolant recovery tank

 

This is just one example. Many catalog stores have these sort of items. Summit, Jegs, etc.

 

Thanks for the info. Was planning on installing that setup. Just trying to figure out the cause of the problem. 

That's easy. It's a classic example of physics at work. As temperature increases, the coolant expands. Once the pressure limit of the radiator cap is met, say 22 lbs, it will allow hot coolant to pass to relieve the pressure.

 

When the engine is first shut off, all the ambient heat remains in the system, but the radiator isn't getting air flow to cool the coolant nor is the water pump circulating anything. So pressure will build until the cap releases and pressure in the system is released. Next thing you know, you have coolant on the ground.

Agree with Frosty,  but JUSTA how old is this cap. Spring and rubber seal does wear out with age.  Installed the catch system on both of my Pontiac's.

JUSTA brings up an interesting point. If your radiator cap is original, it maybe time to consider replacing it with a new one.