Like everyone says, the kingpins must be tight, bushings and so forth must be good.
Of course, figure out what kind of wheels and tires you want to run, paying attention to wheel offset (steering axis wants to meet the pavement somewhere near the center of the tire patch, right?)

Now: ride height and ride softness-
Springs tend to sag- maybe not any softer, just bottom easier.
Rust and crud will accumulate between the spring leaves- it will keep the spring pack from working properly (will act like a solid bar instead of a spring)

The term "rebuilding" could be misleading in terms of the spring pack.
The dropped axle means that you can have a bit more room for axle travel. You can probably go with a bit softer springs (fewer leaves) plus more arch (initially) to give the correct height and a softer ride

Add sway bars to give the truck better roll resistance, and you should be a very happy hotrodder!