If you really want 85 lb spring they should be long enough fully extended to alow for a little over 5 inches of settling to be at ride hight. If you split your 1050 lbs in two that is 530 lbs per spring. If they was 100lb springs, that would mean they would compress 5.30 inches when weight is applied, if coil is close to axle center line. Your 85 lb spring will compress about 1 inch for every 85 lbs you put on it.
85 is a little lighter than I would like, just for the reason of an unstable feel at speed. This will also limit your lower IC settings to be real effective if you wished to go that way.
Softer springs are made for rougher ground, to maintain tire compliance. you won't hardly ever see under a 90 even on a dirt track car, and that would be on the left rear that carries very little weight on a whole.
With your short wheel base you could fall int a group that gives up traction to control wheelies, using lower ICs. I don't like this kind of solution and would rather move weight but some do it. When or if you did, you wouldn't have enough spring rate to off set compression in the rear. Sure you can stiffen shock up but that just slows the compression. It don't shorten it. Then you have traction after the shift when you don't have so much weight transfer. US7race has a spring more to what I would pick.
With center of gravity right and IC right, a spring does little more than hold car up and keep it from shifting weight left to right.