SheetMetal Bed floors

Me and a buddy are doing a new bed floor for his truck before next season. He doesn't want to cut out the old floor, just build the new one overtop of it. What would be the way to attatch the new floor to the old one? The side panels will be welded to the bedrails, but for the floor do we just drill a bunch of holes and plug weld, grind and smooth em out? or is there a better way?