the way i do the rear is, with the trucks frame (about 2-3 inches in front of front spring hanger) sitting on jack stands, i put the axle on the new spring perches,mount the new u-bolts,measure everything,2-3-4 times, place a floor jack under the center of the axle,lightly tighten the nuts on the u-bolts, and jack up the rear of the truck to place the weight of the truck on the axle/springs. now i place a magnetic angle finder on the yolk. set my pinion angle (tapping the nose of the diff with a hammer-up or down as needed). when i have it where i want it,i tighten down the u-bolt nuts, then reach in with mig gun and weld the new spring perches on each side. take her off the stands and shes done.