This is bothering me somewhat as to what is going on. One thought I just had is this. When the car is up on jack stands, wheels off, can you measure vertically from the frame rails down to the springs. Either in front or just behind the axle. My thinking is if one spring has sagged, the dimension ought to be off by the 5/8" you are seeing.
Going back to my own problems with rear springs and as I have commented on many times, I had bought a set of Grab-a-Track 4 1/2 leaf springs. The extra 1/2 leaf is supposed to help stop spring wrap up on my "one wheeler peeler" (open diff). The problem was they were way too high, lifting the rear by 1 3/4 inches. I had the springs de-arched, but then they started to reverse bend. I started to research and only found Eaton reveal the steel type they used. Even Dayton Springs, another huge US spring maker, did not give the steel type. What I did find is some aftermarket manufacturers use a steel know as 4140, whish is a pre-hardened steel used in tool making. It is not heat treatable beyond that. Eaton use SAE 5160 spring steel. That is why I would ONLY suggest Eaton Detroit Springs to anyone wanting correct rear springs.