Here is the GM information packet,
https://www.gm.com/content/dam/comp...hevrolet-trucks/1948-Chevrolet-Truck.pdf about midway through it is the information on springs. It doesn't really tell you waht you are asking, but it does have some good information to use if you go shopping for replacements.
Ask around and find a few other trucks equipped like yours and get a measurement from frame to top of axle. If yours is setting lower, chances are you have weak springs.
If you are willing to get to work, you can pull the springs apart and re-arch each leaf yourself, if you have press it goes easier. Lay each leaf on cardboard and trace the arch, then making small bends, increase the arch in a smooth curve. The next leaf should follow the same new curve you just made. Stay out away from the center bolt area. No heat is needed. Lots of DIY video's showing how to do this.
