I thought about what you wrote and checked the clearance, and can remove the tank without disturbing the engine. So it would be a job I can do in an afternoon for just the materials cost, which should be relatively low (on the scale of boat bucks). Much better to do it as PM than to have a fouled bilge and need to remove a dripping/leaking tank sometime down the road.

So I checked the Ronco catalog and the best fit is 18 gallons -- the current tank is 20 gallons and I'd rather go up than down... There is a supplier named Kracor who makes one 19 gallons that would fit. Any experience with them? Other suggestions?