Sweet build and great work so far! I've always wanted a CCSB 4x4 square body and having a LQ9/4L80E drivetrain is icing on the cake
For the springs, I'd take a look at Office Road Design (ORD). They specialize in stuff for square body trucks and do custom leaf spring packs from Deaver but built to their specs.
http://offroaddesign.com/catalog/ORDCustomSprings.htm