The rear bushing on your SS spring require a (7/8 o.d. x 1/2 i.d.) bushing. A-body shackles bolts are .50" shank compared to the B/E which are .5625" or 9/16" shank bolt and use a 1.0" o.d. bushing.

If you are going to use a longer shackle, make sure they accomodate the smaller 7/8" bushing (Moog K7309 or Rabestos 576-3026) found on the SS springs.

Just curious, why or where a longer shackle is required?