Have you thought about seam welding the shell? Your a welder aont you? So really it wont cost much, you'll need primmer and paint over the bits you've welded but other than that, all it will take is time (and quite a lot). When Nissan bought out the final skyline GTR (the Z tune iirc) they added spot welds the length of the floor pan and around the door frames, you could do something similar to get the shell more rigid, I'd be tempted to take it even further maybe and do all around the suspension mounting points. Will give you a great platform for a good handling car, but will be quite an involved process.