75 Dart - as thin bodied as they say?

the big problem with cars and truck over the years is not the thickness of the metal, but rather the overall design of the body, and coatings and paint that are used--or "not" used.

A good example is some of the mid 70's GM/Chev pickups. The design of the inner fenders trapped mud and water, and the fenders literally rusted out from the inside.

Lack of any aluminizing, undercoating, less/ thinner paint, etc, may have contributed.

This 67 Dart I have has undercoating. I don't know it's "weather" history, but evidently it spent it's life in the Pacific NW. It is virtually rust-free.