Re: Take the bench seats out without compromising the boat strength.
I have seen a couple of boats where the seats have been removed, but then replaced by seat boxes that go only about a third of the way across the boat and are staggered along the length of the boat to allow passage. These are fastened to the sides of the boat and to the bottom of the hull. I don't have a photo to show, but their design connects the boat's sides to the bottom, and are full of foam.
The big problem is in how to attach them to the bottom, and for that I have no answer. Probably a really good aluminum welder could do it, but it's not something I would try. I'm sure it would be far to easy to melt through to the hull. It also might be possible to glass such a thing to the aluminum bottom. I do not know.
Another thing that might work would be a long box structure down the side of the boat, maybe 12" wide, again full of foam and attached to the sides and the bottom of the hull. That would add sufficient strength. It would all depend on the structure of your boat.
One thing's certain: You must have something there to reinforce the structure, or the boat will soon be out of shape.
BTW, what sort of boat is this, a v-hull or jon boat?