Super Stock Springs... how fast?

Just out of curiosity... The ´68 Super Stock Barracudas and Darts used only these springs, in conjunction with a snubber and special shock absorbers, the first years before rear suspension was up graded in several stages, to what it is to day.
Question: What ET. was the best for this original set up?
And if someone with in deep knowledge about the early history of this cars, can tell what the first up grade for the rear suspension was, and in what year, it would be very interesting to know. :)
Thank you.
Stefan