Adding a Rear Bumper

I'm excited because the welder came yesterday and replaced the bent piece of framing on Junebug that had been giving me problems with the floor.  Now the floor is finally level!  He also added two additional beams to extend to the rear of the camper in preparation for the bumper that he will add later.  In order for the beams to fit flush with the floor, the original design is obviously not going to work.  I need advice on what to do later when I reattach the skin.  The previous design had some wood and metal flashing going all the way across the bottom, just underneath the floor.  Then the skin folded around that and screwed in underneath the floor.  Should I cut slots in the skin in order to still be able to fold it under and go around the metal beams?  Or should I screw it into the floor just above the beams, and then attach the metal flashing and wood to extend along the floor, filling in the gaps around the beams?