Huh? Wheel well rubbing solutions have little bearing on chassis stiffness. Unless coil-overs are replacing torsion bars, I'm not sure adding reinforcement there is a primary need, although every little bit helps. Add a roll cage for ultimate chassis rigidity.
I started a chassis reinforcement thread years ago that addressed mainly earlier 911s. Maybe worth a re-read. As the structure matured, the factory increased metal thickness, used high-strength alloy steel, added layers, and eventually employed CAD design and structural analysis software, etc. to increase chassis rigidity.
http://forums.pelicanparts.com/porsche-911-technical-forum/259557-structural-reinforcement.html
Sherwood