Yep, I understand. Sheet metal in that area is quite different between the FB and the NB, so not trivial.
Here is an article about a "Hang 10" Dart where Chrysler took a car similar in structure to the NB and converted it to have a fold down rear seat. I think the "Hang 10" was the Dodge version and the "Space Duster" was the Plymouth version of essentially the same car.
Car of the Week: 1974 Dodge 'Hang 10' Dart