I have done this using the DOW Boyancy Billets. One eight foot by 10" by 20" billet will provide 600 pounds of flotation which is what is required of the top deck. These billets are Corps of Engineers approved for house boats and docks.

Yeah, the reason they stacked them is probably because that is how the old styrofoam blocks came from where ever they were manufactured. The new Dow Billets have a little better boyancy than what was under my 1985 boat deck, which was not much better than old chest cooler foam.

It doesn't really matter the size here as long and you get the required 600 pounds of boyancy needed by the boats design. The boat weights around 850 pounds, minus the 600 for the flotation blocks, which leaves 250 pounds of boyancy needed to float the boat. This is where the Balsa core provides the reamaining amount.

Yes, I cut mine in half and then long ways after that to get a size the would fit behind the seats. I used a crosscut wood saw, but I think you can rent a hot wire melting saw that might be ideal.

Cut out the old fiberglass cloth using sheet metal shears. Remove the old foam, and then use wooden props or sticks to hold the new billets in place. Using at least 50 pound test nylon strapping or cord (knotted or folded at each end), apply fresh resin fiberglass cloth over each end of the strapping. You will use these in place of the old flotation support. Important: Sand and clean with Acetone the hull at each spot where you will attach the ends of the strap with fresh fiberglass. I used a masking tape to hold the new resin fiberglass and straps in place while they bonded to the hull. You can peel the tape off later. Remember, each of the four cut billets will provide 150 pounds of float, so they should have at least four straps each to support them if the boat is turtled. This also would include a strap on each forward and aft end to prevent the billet from slipping into the foredeck or afterdeck on capsize. I hope this helps.