A good fabrication guy can put that thing on a rotisserie and go to town making that thing solid underneath with some extra reinforcements like some nice subframe connectors, torque boxes and panel replacement.
That's would I would do.
I would also mini tubb that thing if I was going to be in there, but that's just me.
At this point I am all about rebuilding the unibody. the decision I have to make is all original or resto mod like you mentioned. Both are tempting! I may be forced in the restomod just because of the amount of replacement and things required to make the repair safe. Either way I am loosing my ***!