In later years, Chrysler (and I'm sure others) added new structure to any non-post chassis (2 & 4 door hard top, convertible). Typically they would reinforce the spring attachment points, and perhaps a frame connector at the rocker boxes.
You'd have to examine a drawing or excellent photo of the bottom of a coupe to tell if any of this is present, and how to add to your frame without disturbing original condition & appearance.
