I've been working to install a torque box on the passenger side of my Mustang. There is one on the driver side but they did not start installing one on the passenger side until '68. The box ties the front frame rail to the rocker panel and makes the subframe stiffer. Paired with the new subframe connectors I should have a stiffer frame.
Some vendors sell the torque box welded together and some sell it in two pieces. Mine was a two-piece box from Dynacorn. If it were one piece, I would have had to cut the welds to separate it to get it into the vehicle. It fits quite well and I'll just need to sand off the paint on the edges and get to welding.