I've done exactly what your trying to achieve, stiffening the body as much as possible without resorting to a cage. I stitched welded the entire underside of the car, torque boxes, lower rad brace, inner fender braces, subframe connectors, and even added braces underneath to tie the torsion bar member to the frame rails. I have even read some guys added a brace in the rear to tie the rear frame rails together. Some have even beefed up the rear brace the shocks mount to on the car. I've also beefed up and stiched welded and gusseted the k member, and added LCA braces.