In the 1950's, 60's and 70's, all car owners were expected to buy a new car every two years. Federal laws required auto manufactures to provide "service" parts for their products for ten years, after that time, you were on your own. The oil crisis in 1973 changed most of that. Reliability became a major part of buying a car - that included not rusting. Some cars did get an anti-rust "dip" back then, Mustang was not one of them. Workers in US auto assembly plants had a reputation for being "sloppy". Over the years, I've got to know several of them, I live four miles from the San Jose plant in Milpitas. Most of those I've known reported a lot of drug and alcohol use by fellow workers. Those "users" usually didn't last, but the cars they got sloppy with, still got sold. I got one (three actually), you got one. To be "accurate" in a restoration, you got to think like a worker of that era, and be sloppy.
Jim