Stiffen Up 69 Dart GT conv.

The torque boxes mentioned above are factory installed on all verts. If you have added subframe connectors where the torque boxes already exist, the problem has to be in the rocker sheetmetal skin or rocker structure behind it.
My best guess... Since there is a welded seam there with some body filler over it, the body filler is what is cracking. This is somewhat typical.
There may not be a structure or flex issue at all, only 20 year old bondo.