Like I said earlier-Coolant pouring out that fast should be EASY to find. I saw a Chevy 350 do that before. Turned out his spring seats had been cut for larger springs and cut into the water jacket on the top side of the head. You could see it with the valve cover removed.
I think you have a major problem as in a cracked cylinder. I can't see a cracked cylinder head doing this. As someone said--pressurize cylinders VIA the sparkplug hole one by one. J.Rob