Another possibility, particularly since it's a Chev, is that you'll need to do a different mod than the one Henry suggested. That's what happens when there's only partial info. If you're familiar with how rear ends line up and mount on leaf springs you may already know this, then again, maybe not. First, the saddles that are welded to the axle housing usually have a hole drilled in them that's aligned with the center of the axle housing tube. That hole goes over a locating "pin" that is in the center of the leaf spring pack. On older Chevs, this "pin" is forward of the center point of the wheel opening. That pin is necessary to keep the axle from shifting on the spring as a result of torsional forces from braking and accelerating. Depending on how far off you really are (don't know if you're guessing at 3" or that's the result of actual measurement) you may be able to redrill a new hole in the mounting saddle further forward the correct amount. If the hole would have to be beyond the edge of the saddle then an extension of some sort would need to be added.

If you're correct about 3" then you'll need to get the drive shaft lengthened. I you only moved 1/2" or less you probably could get away with the existing unit.