Gday doc and son,
i do this sort of thing for a living; but this is how i would attack it. Sit the hull on the floor and support it, remove the deck completly as most of it is stripped anyway.I would make the stringers,and floor out of glass.Glass in the stringers leaving a small gap for the plywood transom, the gap is so you can wedge the ply hard against the skin of the transom using the stringers.Next cut the floor in and dont for get to glass the stringers to the transom.Where the belly tank goes is the only place for ply(use 3/4 inch here) and i put a bog edge so no screws are used.Then just flow coat it and put the deck back on ,all done .Cheers...jim