I've seen torque boxes on ragtops, wagons, 340, and big block a-bodies. There are two pairs. The first pair, that has already been mentioned, is just forward of the leaf spring attachment. The second pair mounts between the forward portion of the "frame" and the rocker panel.
The only advantage I can see to the torque box is less weight. I don't know which is more effective at doing the job of making the unibody more resistant to twisting forces.
http://www.moparmusclemagazine.com/techarticles/body/mopp_0501_installing_chassis_torque_boxes/
