[Brando]

The part that needs the most reinforcement on targas and cabriolets is along the longitudinals. Think of a Targa's chassis as a box on the front, a box on the back, and a wide plank along the bottom connecting the two. If possible have a length on each side go from the top seat belt mount down to the bottom seat belt mount on the longitudinal. That should reduce the "floppiness" you are experiencing.