Spring rate is 1" of compression for every 100 lbs of weight on them. Coils or leaf, both use the same rate standard. A variable rate spring is just that, one end of a coil is softer then the other end. With out a load, the truck rides on the soft portion, load it up, the soft portion is compressed down to the heavier rate portion. A variable leaf spring would likely have a few top leafs arched more so they have room to work before hitting the heavier leafs.
Less spring rate is a softer spring. Average load of deflection is what GM calls the spring rate.
Pull up the GM Heratage site information packet for your truck, usually about have way through the information is the spring information. I would use it to compare any new springs.
https://www.gm.com/content/dam/comp...hevrolet-trucks/1949-Chevrolet-Truck.pdf
