I removed the old 4 cylinder I/O. Used 3/4" ply to reinforce the stern and give the the 3 pieces of 3/4" ply that was used to fill in the old hole something to grab to. I used West epoxy and SS screws to attach the filler plates to the large backing plate, then used polyester resin and glass to coat over the entire backing plate, then 3 coats of gelcoat to seal.