I am going going to order 2 ring and pinion sets(10-bolt) from summit for my 85' V6 and 85' Iroc and an Eaton posi unit for the Iroc.
I don't have a summit book, so I don't know what set I need. Do I need the 28 spindel or the 30 spindel set?

 

Spindles are what your front rotor/hub assemblies are mounted on, the differential doesn't have spindles. Are you asking about Splines? The only splines used were early (1982-~1989) 26-splines, and late (~1989-1992) 28-splines. The only thing in a 3rd gen to have 30-spline was the late (late 1984-1992) auto trans.

It wouldn't matter what spline you have to order a gear set, only if you were ordering a carrier/LSD unit. Best way to find out is to pull the axles, someone could have already swapped them out. It is a common thing.