I'm thinking you guys are right. The engine is in a really tight place in my old Columbia. Plenty of room around it, just no access to the top unless it is pulled out. Guess it's time to break out the tools.
Ugh!! sounds ugly. Good luck and let us know what you find. Before you rip it out you might want to do a compression test just to make sure, but as I mentioned before I can't think of anything else that would cause the cooling system to pressurize that fast? Especially after it overheated.