Fanus, moving the axle under the springs is quite a serious lift, as it will be half the thickness of the axle tube plus the thickness of the leaf pack. I reckon that must be close to 6"?
It's a relatively easy mod to do, but you will have to extend breather, brake pipes and load-sensing valve bracket. More importantly, to my mind, that will probably put signifiacnt extra load on the universals, and the amount of work you will need to do to the front end to match this is significant. I think this is probably too much for caster corection bushes, so you will be looking at custom lower control arms or drop boxes, as well as adjustable panhard and drag link (which will also cause bump-steer with no way of rectifying it).
As for re-tensioning leaf springs, do this only if you like to have problems. i don't think there is a single spring works that does this kind of work that has accurate enough control over their heat treatment processes to gaurantee the outcome. I personally would rather look at extended shackles than to do this, but first prize is a proper 2-3" lift kit with specially-made leaf springs.
Mag ons ons kenniskry met lekkerkry aanhoukry.