Looking at the chart it looks like the fj60 rear springs would give similar results but you would get the benefit of the bigger fj60 bushings with the fj60 springs. 55 and 40 bushings are the same right? I go through 40 bushings pretty quickly offroad.
It might be because I am sua with the fj40 bushings. If I were soa with short shackles the fj40 bushings might last me longer. But I would still pick a spring that allows for nice big bushings if I were to start over either way.