The drawing that TDF provided should be very helpful. As far as the marine sealant goes, if the rivet goes through the hull where water is on the other side, yes you want to seal the rivets. If you don't have to go through the hull on the outside and the rivets don't come into contact with water then no. Hope that helps. If you stop short of the bench seats so there is room to sit and face forward you will have to get creative with the bracketing you come up with and will likely have to have fasteners that go through the hull.
Frey