[chopper]

Not sure what type of expansion tank you're running on your Chevy but the Ford FE types have an internal horizontal baffle about halfway up the volume of the tank. The fluid level is brought up to the level of the baffle, leaving an empty space (called the "ullage", FWIW) into which the fluid can expand. Not trying to be cute, but that's why it's called an "expansion tank." It's supposed to have the empty space above the fluid. The expansion comes from thermal expansion of the fluid during normal operation, and also boiling of the coolant in the cylinder heads after shutdown. Most of us who have FEs and live in the warmer climates have some type of recovery tank added to the system to minimize the problems which you have. Make sure you have the later type of radiator cap which has the reverse check valve so that as the engine/coolant cool down, the resulting vacuum will draw the coolant back into the expansion tank.