64 273 new rebuild now with coolant in #6 (It's a Hybrid!)

You should have pressure checked the cooling system before tear-down.
While thats a good way to see if there is a leak...it wont pinpoint it.. to say 'a cracked head or gasket'...it just tells you whether or not the system leaks in general.
He already knows it leaks by the coolant filled chamber.

Pressure test the head itself once its off , to see if its cracked...disassemble and mag it to find the cracks.