As long as you are not cutting the spring perch off the axle, I doubt your going to add enough spring lift to change the pinion angle much, unless you interchange the 4x4 axle for the 2x4 axle. The 4x4 axle should be mounted under the springs, and the 2x4 axle should be mounted above the springs, swapping those would really change your pinion angle.

The thickness of the spring leaf you are adding is as important as the length. Thicker leafs add more capacity.

From your extra spring packs, add a matched pair (read that as 2 the same length and thickness) of long spring leafs to the truck that needs more lift, and a pair (or 2 pair of different lengths) of shorter matched spring leafs to the truck needing more capacity. Don't forget to pick up a couple pairs of spring center bolts, and I would also pick up a couple pairs of the spring alignment brackets to add at the front edge of the newly installed extra springs in each pack. Gene