The oil available in the 1970's when I started driving my first cars was far different than what is available today. I'd say from experience that "good" oils didn't start to arrive in the market until sometime in the mid to late 80's, although you could see advanced oils show signs of performance in the late 70's, it was mostly along the line of the first decent 5W~30's versus 10W-30. I expect that the products available in the 60's were even more basic as far as multigrade VI improvers go.
STP has a bad rap, some of it deserved. But with high wear engines it actually did help keep them on the road at a time when 100,000 miles was considered the life of an engine; getting 120~ or 130~ was considered a bonus. As I understand it (anyone with better information feel free to chime in) STP is mostly VI improvers skewed towards a high SAE rated viscosity ... maybe 50 or even 60 weight. It looked like molasses out of the can and even after it was diluted in the oil you could always tell it was there because it created a sticky finger feel off the dipstick, so obviously it's sticking to metal.
The poster who said it was ZDDP heavy doesn't surprise me if true. Catalytic converters and zinc was not an issue in those days, even when they became more common in vehicles prior to perhaps the turn of the century.
Modern oils don't need the help. But the 351 and 429 are not modern motors. Only a cam change and a compression ratio adjustment away from being in a bus or a passenger car (performance cars would have more internal changes, but your sedan or wagon, not so much, and in performance applications they didn't last as long as the more mundane sedan and truck variants).
Did adding STP hurt longevity? I never saw any evidence that it did. I've never added it to one of my vehicles, but it was a common practice to find it in the high mileage cars high school students could afford to drive back then, and I've pulled a lot of dipsticks (worked at a full-service station all through high school) and it was pretty common to find evidence it was in motors.
