SOA will increase the ride height by the spring pack thickness + axle tube diameter + old perch height + new perch height. It will probably be around 4-1/2" to 5". SOA is always a good mod, since it increases under-axle clearance. If the resulting lift is too much, a shorter shackle can be used (no shorter than stock though), andd/or leaves can be removed from the pack.
If you remove leaves, DO NOT remove the overload, as doing so will cause serious spring wrap issues unless you add some other axle wrap control. Don't eliminate the leaf guides either. I will probably do a write-up on leaf spring pack mods here in a few days since I'm working on that very thing right now. Keep in mind that fewer leaves reduces the overall rate of the spring, which means that a given amount of load or tongue weight will sag the ass end more than would be the case with a full pack. On the flip side, it will have a softer ride.
If you do an SOA and need a pack with less lift but you want the same spring rate, then you are going to need a custom pack (or build one from donor packs). If you are planning on getting new custom springs anyway, I would definitely do the SOA. You shouldn't even need to remove the old perch from the bottom of the tubes - just get two new perches and weld them on after you set your pinion angle, and add a couple shock mounts. Any fab shop should be able to do this if you can't do it yourself. For a given ride height, SOA will always have less arch in the spring than an equivalent height SUA, which is ALWAYS beneficial.
Sorry, couldn't help it!
