I did the same as " Builder ". Set the boards in the correct position on the cross rails, then set your box sides. Mark the holes, disassemble, drill, finish & waterproof the boards including the holes, and reassemble. On my '40, the front panel on the box has a flange that the boards sit on. You can mark the hole locations from underneath. The rear cross sill also has holes but these ones fasten thru the bed strips.
