Deck is 24' long, right? At the 12' mark have welder weld in metal to widen the mounting point to the crossmember, use 12' PT lumber, personally I'd go 2x6 from each side and trim center board to fit. To remove current floor, use a sawzall on cut boards at each crossmember on each side. Use a hammer to break the screw/board left on top of the crossmembers. Go back with 5/16" diameter trailer deck screws, 2 per board on each crossmember, its worth the effort. As far as the, oil/diesel board treatment, waste of time.