ok well the length of the spring should be correct for the shock you are running. Take a shock off and remove the spring and measure the extended length and the compressed length, center to center mounting point to mounting point (rod end, clevis, what have you) this should give you enough info to go to the shock maker and find out what the correct length of spring is, also make sure you check the dia. not all coil overs use the same dia. spring. Then select the spring rate based on the weight on the springs and how much shock compression you want at ride heigh keep in mind how much spring pre-load adjustment the shocks have.
If this is a ladder bar car with adjustable upper links of the ladder or a four link car I would not recomend trying to control "body roll" with the rear springs, there are better ways.
A word of caution ....if you install a spring that is not compressed or worse has space between the adjuster and the spring when the shock is fully extended you could have a problem if the suspension goes to full extension when the car is moving.