Randy - going to be tough to pull that complete inner framework as a unit without causing even more problems. While it might be spot welded to the lapped over inner side skin, that weld wont go through to the outer skin surface. Spot weld cutters use a center drill to locate and hold the tool in place and a separate cutter to mill the actual spot weld off. This of course leaves a fresh new hole to fill - and you still haven't been able to pull the skin off of the frame. You will then have to pry that outer skin off of the frame, possibly (most likely!!) distorting it - and a further problem trying to straighten twisted and probably stretched outer skin metal.

I think I would approach it in sections rather then as a whole. Where you want to make a repair, using your die grinder and a carbide bit (or a reduced diameter cut off disc) grind those spot welds then cut the actual inner framework at that area and carefully pry it out, distorting as little outer skin as possible. This leaves only that smaller selected area for repair, you then can MIG that smaller piece of removed framework back in place and reducing the amount of visible area that was repaired inside and minimal finish work outside. More time consuming - but at this point, it only amounts to a few hours (on top of even more "few hours")