I had ESPO springs on my '73 wagon (had because I recently sold my wagon). I put them on it about 20 yrs ago. Before I bought the ESPO springs, I had a set of springs off a factory big block 4 dr. The 4dr sat a little high in the back with just the springs, so I figured they would work.......nope. Swapped out the springs, and it went straight to the bump stops.
With that being said, and this is not a complaint, but, I bought the 1 inch over springs and they pretty much leveled out by the 5 yr mark. Normal for new 1 inch over springs to do that? I wouldn't think so, but they never got worse and I never looked into it.
The only installation issue I ran into, which really wasn't and issue, was the locating pin for the saddle was for '72 back not the '73-up iso clamp set up. I just swapped them over from my old springs.
EDIT: when I bought my springs ESPO had a comprehensive catalog online, now it looks like you have to call in, but they do show B-body springs....66-73 Belvedere, Charger, Satellite, etc. (B-body) $230-$280/pair