Spring advice!

[1972 Banana]

I am running a 6 inch lift with blocks in the back. I sent an E-mail for some tech advice at one of the offroad dealers and he told me to run 8 inch springs in the rear and tha six in the front due to the weight of the Blazers. I am planning on installing zero rates to center my axle up as it is not centered now. I am thinking this will be to much as I do not even run a hard top on my truck only a soft top. any advice would be appreciated, he also said to order the 52 inch spring! any insight to spring size as well?