My '69 Barracuda 340 developed rear wheel hop on hard acceleration only a few years from new. Maybe the rear was sagging also, but I didn't measure it. Had the springs re-arched, and the shop also added one more long leaf for a total of 7 each side. This fixed the wheel hop, and the rear sat maybe 1" higher than spec, which I liked.

Today, 50 years later, the springs are doing fine and still sitting about 1" high. I suspect that the extra leaves by themselves is mostly what fixed the problem, and that the re-arching didn't help that much.

My recommendation for sagging rear springs is to get a pair of used 2 1/2" wide leaves slightly shorter than the spring eye to spring eye distance, thickness the same or slightly thicker, redrill the center hole as necessary with a carbide drill, re-assemble with new pure, non-magnetic zinc interleaves (don't buy fake, magnetic galvanized steel interleaves) and new clamps, and see if the problem is fixed. If so, the cost of re-arching is saved. If not, then get the assemblies re-arched along with the extra leaves.

But be sure the re-arching shop doesn't throw away your pure zinc interleaves like my shop did.