I have a '64 Valiant Conv and the lower rear was trashed. The only thing that will fit on your car between the roof piston housings is a Dart or Valiant convertible rear seat. Maybe some other body family will work but I never found any to test. The conv seats are about 6 inches narrower on each side. I will attempt to attach photos of my fix.
I used the rear bottom from a hard top and cut it down to fit. It wasn't easy dealing with the spring steel but after searching around seemed to be the only way.