Layout question

[Bushwacker]

That inner liner adds a lot of torsional stiffness to the hull which will be lost when you remove the solid inner walls, but I think the best way to restore that is to core the hull. That will uniformly stiffen the entire hull without creating local hard spots and the resulting stress concentrations that you'll get with local ribs. Might cost a bit more in materials, and the ribs would probably work ok since the hulls are so overbuilt to begin with, but the cored hull would be structurally superior in terms of torsional stiffness and rigidity.