so you're looking for 12" of total lift!? 6 in the SOA and 6 from springs?
the only excuse for that is 50"+ tires and WIDE axles
if i were to run 39s on my MJ, I would do so with 8-9 inches of lift, there is no unibody strength in any of the MJ fenders so you can just cut away. I have 8 inches under my MJ now with a spring over and a mixed assortment of junkyard leaves.
if you really want 6" spring packs, I believe you will need to get them custom made, Alcan comes to mind