Well, the cars they ran then started out life as a stock car. They weren't exactly a true stock as in off the lot stock cars.
You are correct but they at least started as cars made by the manufacturer. They were not custom race chassis with a replica model body on them. I have seen many of them in collections, and you can clearly see the bodies and chassis were of the heritage from the car brand.
I wasn't around to witness those cars firsthand in the 60s & 70s. However, I guess it depends on what the definition of "custom race chassis" is. Even back in the 70s they may have utilized the factory frame rails but the cages, rear diff's, front spindles, steering components, brakes etc. were all being modified or custom built. Granted, there were a lot of parts that just weren't available by calling a manufacturer compared to today. IMHO opinion those modifications make those cars much more custom or purpose built than most people like to admit. There really wasn't much "stock" about a stock car, but they were more stock appearing. As far as OEM support, I'm pretty sure even into the early 90s, Cup cars were required to use factory stock hoods, roofs and deck lids to meet body templates made to OEM shapes.
