I'm working on a 58 Impala posi rear differential that has no posi effect. It's a factory stock unit. Each rear wheel turns freely independent of the other. Disassembled it, don't see any obvious excessive wear, the clutch plates look ok, each side has 2 toothed to the case and 2 splined to the side gear retainer as shown in the 58 shop manual. I see in later years (found a good 63 video) everything looks the same but GM included a wave "spring" plate in each side. I see in the 58 shop manual, the clamping force is measured as clearance at the spider gear shafts to case and if too great the recommendation is to "cut shim stock in thickness required to bring clearance within limits". The shim stock being added to the stack of clutch plates to reduce clearance at the spider gear shafts and provide greater clamping force from case to side gear. Is there an updated clutch pack produced for these differentials? Would the later 63 set of plates with the wave spring plate work in this earlier carrier? Any help or discussion of experience with this issue is greatly appreciated.