I helped a buddy lift his 5er. We build a subframe out of box steel 6" tall. (maybe it was 8")
Anyways, we cut off the hangers, welded up the frame extension to the bottom of the existing frame plus boxed it in by connecting left to right side, got new hangers and welded those up and then reinstalled the shackles, leafs and axles. (Tandem axles) Works great! Oh, we had to extend the brake wires due to the extra lift.
It helps that my buddy is a machinist and we had all the equipment to lift and block the 5er as we did the work. I learned a lot!!