Thanks for all the replies and comments.

In the end, I suspect I may have to replace the exhaust manifold/coolant reservoir, but before I do so (at a cost of $500), I think I am going to try to CAREFULLY open up that bronze flap a bit with a dremel tool, hopefully, exposing that end of the hole to where the elbow screws in. I'll then try using a stiff wire to break through the mineral build up I suspect is clogging it.

When I was at the boat the other day, I attempted to remove the Exhaust manifold/Coolant reservoir so I could take it to a radiator shop for some expert help. I was able to get the 8 bolts connecting it to the block off without problem as well as all but one of the various hoses, but there is a short piece of about 6" diameter rubber exhaust hose clamped onto the outlet that I was unable to get free (photo showing it is looking aft, along the manifold).

If I have to, I can cut it off and pray that I can get a new piece installed when it comes time to put everything back together. Next time I'm at the boat, I'll try using a heat gun to soften it so I can get it off, but I don't have high hopes that will work too well as after all, this is an exhaust hose and routinely exposed to hot exhaust gases. If anyone has any other suggestions, they would be welcome.