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 !! Maybe check Ebay,Year One,I know they're out there,as I was very tempted to put them on my Dart just for a little added insurance !!