Any '64 -'72 Chevelle rear will bolt in, the earlier ones (pre-'66 I believe) will be narrower. Another good option would be to find a '71 -'72 Olds or Buick rear, they are the stronger 8.5" 10-bolt rears.
A good Chevelle 12-bolt is getting harder to find these days. One thing your friend might consider is a spankin' new one, available from
Moser Engineering in pretty much any width you choose. Even a Ford 9-inch is worth considering, they are available to bolt right in as well.
All the '64 -'72 GM A-body rears will swap from one brand to the next, nice that GM cars have that ability.