I put 1/4" strips of aluminum every 8"s or so with some center braces for the middle i got 1/16" checker plate to go down on the top of it ill be just putting sheet foam inLooks like a great project! I'm in the rebuilding phase of my 1984 Islander i/o to outboard bracket conversion. I really like your idea of putting the solid strapping / bracing across the stringers. That will really lock everything together and give great support to your floor. What thickness of aluminum are you putting down on the floor? I'm going with .125 for mine. Are you going to do sheet foam or pour in foam for flotation? After taking out close to 900# of waterlogged pour in foam, I'm going with sheet foam.