What I would do is find a '73-'86 K series or '87-'91 V series truck or Suburban as a donor for the springs and differentials and possibly the transfer case. A couple of the better transfer cases to use would be the NP205 or the NP241. Those are both married transfer cases, which is what you will want to use. To bolt either of those to the back of a 4L80E you will need an adapter from someone like Advanced Adapters. I would not consider a NP203 as they are very heavy and usually need more work than the other transfer cases.
If you were to find a donor as I mentioned above, you would not have to find an original K series Suburban. Take a look at Jon's build thread here.
http://www.67-72chevytrucks.com/vboa...d.php?t=448681