Talk to Sid Drapal at Nostalgia Sid's Dropped Axles,
https://droppedaxles.com/ He knows what he's doing, and can get you pointed in the right direction. I don't know what all exact reasons putting the axle over the springs is a bad idea, but if it was a good way to do it, it would be commonly done. I can only guess that the handling is questionable at best, and the size of the axle on top of the springs would limit suspension travel.
My truck has had a 3" axle done by Sid that was installed in 2016. With the proper caster and toe settings, and fresh tie rod and drag link with modern tie rod ends, it tracks beautifully and has ZERO bump steer. Highway speeds and two fingers on the wheel.
https://media.fotki.com/2v2H55bnWxDubp.jpg
