Hi Michael,
Everything you have done matches the results I obtained when trying to solve the same problem. When I switched springs from side to side the measurement difference did not change and with the different springs in place. This made me realize that the difference was not a function of the spring but something else.
When measuring height from side to side, I measured body height from garage floor to top of wheel opening arch directly over the center of the hub/knock-off. I bring this up because this measurement reflects the positioning of the fender and includes the difference in fender adjusted installation to the body as well as the spring-imposed differences. Although this is not a major component of the overall height measurement, it should be understood as being present and could be as much as 1/8" on a reasonably straight car.
When reviewing your numbers, it was quite interesting that at some point both sides did become equal. I would interpret that to be the result of lessoning body weight across the rear and lessoning dependency on the springs. Jacking could also be pushing up on the rear and shifting downward pressure on the front springs/suspension. Keep in mind that over the years our sheet metal understructure will sag and deform under age and stress to become less then side-to-side symmetric. As a result, matching one side to the other will be an action of mismatching side-to-side to gain optical symmetry.
As I see it, an indication of supported height imposed by each rear springs can be found by measuring ground-to-spring at each spring attachment point and, although measuring wheel-arch measurements present optical symmetry, it includes too many factors to understand and determined what to change. When I did this I found that front spring mounts were within 1/16" of each other and rear mounts were equal. Based upon the fact that my body height was sufficient as a result of adding an extra leaf, this result indicated to me that spring support was not my issue but other factors were causing the optical difference.
Because the axel is mounted on top of the spring, spacers will lower rather then raise body height. Since body height was raised by stiffening the spring, adding a 1/4" wood spacer to the higher (right) side lowered that side to 1/16" of the lower (left) as measured at the fender arch. As a result, my optical symmetry was reasonably achieved. Over a year's driving the wood spacer had cracked and some change has taken place. To address this spacer deterioration, an aluminum spacer will replace the wood within the next warm days.
As mentioned, the front suspension has an affect on the rear and vis versa. Although I would like to investigate the effect of changing front suspension heights, this would complexify the problem far beyond my minimal equipment.
Last, having reached a set of rear springs and found them to be short lived and a waist of time, money, and hope. I would never recommend this activity. However, I must suggest that my disappointment could have been the product of the servicer and not the operation.
Hope this helped,
Ray(64BJ8P1)