Stiffen Up 69 Dart GT conv.

Of course I can't find what I'm looking for,but there are Factory plates that weld into 4 corners of the subframe to the floor pans that were used on the convertables for just what you were talking about,flex !!

Those are torque boxes, rag tops & big block cars came from factory with them. His flex is coming from somewhere mid chassis. On a frame machine one can quickly and accurately measure all frame dimensions, and ascertain problem areas. If the inner rockers are rotted out, some sort of a jig will needed to square up chasses to factory specks before welding in structural parts, a frame machine is ideal for this work.