The reason cars started to rust back then, was because the steel companies were using the new Basic Oxygen Furnaces to make steel. The BOF were a more efferent way to produce steel, over the previous Open Hearth method. OHs used fuel oil, while the BOF did not use ANY fuel. By blowing O2 into a vat the impurities in the mix became the "Fuel" to create steel.
It was quit some time before the steel companies realized not enough of the impurities were being burned off.
In other words the rust started within the steel itself. The cure was simple, they simply blew the O2 for a few more seconds.
GM used steel from US Steel Corporation and Ford use steel from Bethlehem Steel. GM developed the problem earlier than Ford because Bethlehem Steel started to sell BOF steel a year later than US Steel.
Search BOF and OH steel making methods
The hot dipped galvanizing can not be used for exterior body parts because the finish in not smooth enough for panting, the steel must be zinc plated. Take a look at a mop bucket to see galvanizing look like
When a car is welded together, piece by piece, and gets ready for painting, the steel starts to oxidize (FeO)just sitting there. The body must be acid cleaned (ex: dipped in sulfuric acid) to clean off the oxidation just prior to painting. Priming and painting, and even adding rust resistance) after it starts to rust is like closing the barn door after the horse gets out. The rust spreads and works its way through the paint job. Subaru in the early 80's was notorious for this. So were certain VW's, etc.
Chrysler, through Lee Iacocca (sp?) did the best job overall regarding rust. They switched from bare, low carbon steel to hot dipped galvanzed steel for the exterior body panels. Their cars in the mid
80s were the most rust resistant cars made back them. Being too expensive, it seems no mfgrs use this process any longer.