Scotty, welding the frame is easy...the problem is that the box sits tight to the frame when it's done. If the box were "close" to the frame you could shield it from the heat, but there is no way to weld the frame right up to the underside of the box without catching the box on fire. Unless you didn't weld the frame seam on the top, and who wants a repair like that?
The box has to come off of the frame for the repair to be done correctly, and the ins co is likely to total it based on the cost of doing that. I wouldn't trust any frame shop that said they could repair the frame right with the box still in place. My $.02.