As best I recall, when I have done it in the past, I positioned the pads approximately where the old pads were, but placed new pads at the new spring-pad mount width. Next, made sure the housing was centered, then mounted the U-Bolts and Base Plates, snugged U-Bolts and lowered the car to the ground; Next, rotated the Pinion to the correct Pinion Angle, Tac-Welded the Spring Mount Pads, then lifted the car, removed the rear axle housing, then fully welded the spring pads to the axle housing tubes.

Finally, remounted the rear end housing into the car and verified that the pinion angle was still correct.

Worst case, I would add slim wedges to correct, but it should Not be necessary.

I've done it three times (8 3/4 and Dana 60 for my '71 Charger and the Dana 44 in my '79 CJ 5 Jeep) and only one needed a pair of thin wedges to get the angle spot on.

That said, there may be other or better methods.