On my 1990 V3500 454 TBI I had a similar issue which turned out to be the magnets in my pickup coil were broken up and causing random firing issues which manifested in surging idle. So take you cap off and see if you have any pickup coil failure/degradation.
I can say with 100% certainty that these particular springs have imperceptible wrap. 10,000 lbs. in the back and the truck drops 1/4" and I have purposely watched under load the spring reactions.
I didn't go with springs because it's a work truck for me (note dump insert). I have special 9/1 5,000 lb. rear springs on the truck. It handles 10,000 lbs of load like it's 10 lbs.
I have a 1990 V3500 that I did a 4" lift on. I bought the pieces separately because that just happened to be the cheapest way. 4" front Superlift springs, 4" Superlift rear blocks, 4 Superlift shocks, 1 3" Dana 60 Steering block. You also need to space down the driveshaft center bearing like...