1969z28rs

I have a 69 Z with it's original drive train. After turning off the engine, it seems to always build pressure and push coolant out of the overflow hose. Even with the radiator being half full of coolant, it still has enough pressure to force the coolant out. I am pretty sure there is not an air pocket because I recently had a new thermostat and coolant put in. Any ideas why this is happening?