I still say sometime in the mid '60s, not much was "stock" anymore. I Have known this since the '70s when I first became interested in NASCAR. Various articles you can find will agree. Here is an example:
"...In 1966, Holman/Moody built the first “half chassis” car by grafting a Ford Galaxie front clip to a Fairlane. The next season the front clip would be tubular, and soon after that NASCAR wanted to shift the focus to the driving and the drivers, rather than the cars. Every NASCAR stock car had (and still has) a 5.8 liter (~350 cubic inches) naturally-aspirated pushrod V8, 4 speed manual transmission, and a solid rear axle (non-independent rear suspension). The last road car with this configuration was sold in the early 70’s. Ford’s last year for the big block/4 speed/floating rear axle was 1971.
During 1970’s NASCAR teams had been running fully custom tube frames for a few years. They had very long hoods, mid-mounted engines with the driver’s seat closer to the rear axle than it would be in a production 2 door/4 seater car. The new GM, Ford and Chrysler models for the late 70’s-early 80’s were front wheel drive, small 4-cylinder and V6 engines, much smaller unibody construction not at all suitable for high speed oval racing. Fuel consumption and horsepower were way down, and the strong nodular iron-cased 4 speed manuals were traded out for lighter-duty Japanese made aluminum cased 5 speeds.
