Stiffen Up 69 Dart GT conv.

The structural inner rocker panel may be degraded from unseen corrosion allowing body to flex too much. A lot of moisture finds its way into the rocker area in a convertible via the top’s gutter system. The inner rocker is what connects front and rear sub frames, where convertibles don’t have a solid roof I believe that inner rocker is of a heaver gage metal then sedans.

I would get the car onto a frame machine, and figure out what is sagging & flexing, and then repair it before the car deforms any further.