Looks like you notched the frame brackets to clear those rivets (duh) but my conclusion is that you bolted the brackets to the frame and then slid the C channel on them then welded the C channel to them so now you'll never get that bumper off without cutting it apart?
I thought the same thing too. Easiest way to get it off is to jsut weld a small tab with holes in it and use a chain binder to pull the back ends of the plate together so it clears the rivets. should pull together just fine to clear.
