Marty,
Sell the car, too many variables in the rear springs.
C7ZA-5556-AM is listed for:
67/68 F (63) 8 cyl, 390 for competition handling GT only "From 11/15/66"
67/68 F (76) 8 cyl, 289, 390 for competition handling GT only "From 11/15/66"
68 F (GT350/500)
4 leaf-665 lb. load rate-#C7ZA-5556-AL,AM
Service number - C7ZZ-5560-T
Jim