Thats probably the way they are. I would think if the top is up and the car drives nice and tight you answered your own question, assuming everything is mechanically ship shape.
Convertibles flex, I had a 66 Vette roadster, never drove a Coupe but Duntov wanted to build only Coupes in 1963, much stronger car.
More specifically I know a club member that had a beautiful 66 Eldorado, he was never happy with the stiffness of the car.
There is no torsion bar to adjust, enjoy the top down ride.