John can answer this better than I can, but the spring rates themselves were based on what options were added (or subtracted) to/from the car. In general, different springs rates were used to keep the "ride height" at a constant, compared with other Camaros.
With a few variances between years, this height was based on two basic conditions:
1. "As Shipped", which was the car, oil, water, 2 gallons of fuel and the recommended tire pressure (24lbs front and rear)
2. "Curb Weight", which was the car, oil, water, a full tank of fuel and the recommended tire pressure (24lbs front and rear)
For 1967, the measurements are on UPC 3, sheet A8. For 1968 you can see the measurements on UPC 0, sheet A11, and for 1969 it's on UPC 0, sheet A10.
There were a lot of other factors involved, but the above is the jist of what they were trying to accomplish.