Here's an earlier thread on the topic:
http://67-72chevytrucks.com/vboard/s...ight=rear+axle
And here's a tip - when redrilling, one or more of the existing holes need to be plugged. Rather than puddle welding the hole, the guy that did mine sliced up an axle stud keeping only the splined portion. That was pressed into the hole and tacked in place, avoiding excessive welding that could warp the flange.