mike.d, Guy and Nicholson 58, thanks for the comments.
The leak occurs when topping off the tanks
(they are piped together) and only when near full. The overflow tube is connected at the top of the tank and passes through a longitudinal bulkhead, emptying into the mostly enclosed anchor
chain locker - not enough room to get my head
in there either. Pressing up the tank may still allow the overflow hose to push air and water
out, but if it's loose on the top of the tank, it may also be the cause of the leak.
So it's either the tube is loose from the tank (factory pics I took show two hose clamps as fasteners) or a crack at the top of the tank. Either way, I'm going to have to go into the space and just trying to determine how big an access I'll have to cut.
I'll check out the other sources you all mentioned for other scopes - thanks!