Have this problem on one of my Sierras, I think the beam mounting bolts have been over-tightened by someone in the past and its broken the spot welds on the nuts so the bolts don't tighten

Anyone had this problem before, looks like I'm going to have to cut into the shell somewhere to fix it, the question is, is it easier to do this from inside the car or underneath it (I don't know where the bolts are located specifically), but its one of the long bolts (the main one that holds the beam on) and one of the small ones on the beam mounting plate that won't tighten. I'd rather know what to do first before I go attacking it with a grinder in the wrong place