hard to tell from that pic, a zip wheel on a die grinder might get it off, but it's highly unlikely you'll cut a welded fender loose without messing up the paint, including on the other side of the metal [inside the box] .... if the fenders look OK as-is, I'd be more inclined to remove the bed, with fenders, and flip it up to do the work you want, much less hassle and refinishing - and welting on the front doesn't mean there was welting on the back
Bill
