The vintage Mustang in most folks' driveway is not the most flex-free platform in the world. The Mustang has--since its inception--been a unitized platform. This might make for a lighter car, but it also makes for a vintage chassis that is quite flexible when it comes to the whoop-de-doos and potholes. In some cases--particularly the convertible--this has been readily apparent right up to the SN-95 chassis. It is often quite disconcerting to feel the body undulate across bumps, or watch in horror as the gap at the leading edge of the door changes as the car is jacked into the air.
Ford saw that this would be a problem in 1963 as the engineers designed the chassis for the convertible. So they placed torque boxes under the toe board where the hardtop and fastback received none. The factory also added additional bracing along the doorsill, as well as across and under the tunnel of the convertible. This helped some, but the car was still not as stable as the hardtop. All that being said, we sat musing over what-ifs. What if you took the torque boxes that the convertible used and put them under the toe boards of a hardtop? Would this help stiffen the hardtop body?
We posed this question to Merv Rego at Classic Restorations of Central Florida, and his answer was yes. So since Project '66 needed floorpans and toe boards on one side anyway, we decided to invest in the torque boxes from KS Reproductions. The installation is not a simple one, but the enthusiast who has tackled floorpans before should be able to handle this with some time, effort, and careful measuring, as well as prudent cuts.