Sailor-
FWIW here is what I did in 1985 to my '76 Seafari when buying a new Merc with an oil tank. I glassed up a 5/8" plywood tray and glued it down to the centerline stringer and the hull with some polyester resin thickened with cabosil; been there ever since. A few years ago used it to attach another bilge pump. The tray freed up some volume under the stern jump seat.
Best wishes
