Centering new rearend ?

You can measure all day and when you put it under the car you will find it closer to one side then the other. I have found the only way to guarantee tires will be in middle of wheel wells is install axle with pads not welded on axle under car with tires and wheels.
Move till you have tires same distance outer wheel well to outer wheel well Set pinion angle at this time, tighten u bolts to spec. set at ride height to be sure all dim. are were you want them. Then tack weld pads in place, remove axle and weld. If your good you could do it in the car. Note: when you narrow a axle to get the correct width put tires and wheels in wheel wells (no axle in car) car at ride height, put shims, wood, cardboard, or to keep wheels in place. Now you measure inside of wheel to inside of wheel and you have the width of axle, flange to flange. don't forget pinion off set, Moser uses 1.5 for Mopars.