Looking at the pic, I believe that the way that bumper is made it's actually covering the cross sill. The one that goes across the rear is flat, not an I-beam, as it also is the rear of the bed floor. When I said that it attached to the bedsides, I meant that it attached at the stakes. I apologize for not being more clear with my descripton. Looks like you may need new stake pockets too. Check with Mar-K. They can set you up with the needed sheetmetal.
Oh, and the round thing on the side? I think it's a holder of a fire extinguisher. I've seen small fire rigs and service vehicles with the same bumper and with the same ring on them.
