You've posted a Tech question in the parts for sale area so may not get a lot of input, but the chart below should give you information on stock spring rates. Those I'm sure have sagged a good bit from when they were new so probably aren't the same as what they were rated for. A set of 100 lb rear springs should tighten up the handling, handle the extra transaxle weight and still work well for a driver's car not a track car. If you'll be tracking the car, may want to move up to a 120 lb spring, but it will be quite a bit stiffer on the road.
