I bought a '69 Camaro from the original owners around 1976 or 1977. They had bought it new when they lived in Los Vegas NV. At the time of purchase we were both living in Santa Maria Ca. The car had always been in a dry climate and never undercoated. The under carriage was mostly gray as I recall with very little overspray except in the area of the right rear quarter panel that was replaced because of an accident. This particular car had a chrome molding that ran between the front and rear wheel opening just below the door. The area below that molding was painted a semi gloss black with almost no overspray showing. The rear wheel wells were painted with an undercoating type finish and the front wheel wells were the same OEM type semi gloss black that Chevrolet used at that time on their inner front fenders with no undercoating type treatment on this particular car. I don't know for sure what Chevrolet did on cars going to other parts of the country, but most of the cars I remember were treated in the same way and my thought is that it may have had to do with where they were shipping the car for sale. The eastern and northern parts of the country it would have made since for them to add additional rust protection in general and they did offer an option of undercoating from the factory, but most dealers would order their inventory untreated and then sell undercoating as additional mark up and profit. One thing to remember from that era was that the old adage of "never buy a car built on a Friday" held truer than you might think. The quality control of that time was far more hit and miss than we see today. In the end you're looking at a machine built 50 years ago and the finishes weren't consistent back then. So I'd build it the way it looks best to you and call it a day.