You could also try to find a 14 bolt rearend out of 73-87 Chevy truck. They had a dually version that was 63" wide, which is the same width as your 3600. Mostly, they came in a 4.10 gear ratio, but 4.56, 3.73, 3.42 and even 3.21 are out there.
If you want disc brakes, you can look at late model GM from 99 and newer trucks and 2003 and newer vans. The late model rearends are typically wider than their earlier counterparts, it is possible to retrofit their brakes to earlier axles.
For front disc brakes, well that's a tough one. If you stick with the 8 lug, you're not going to find much. The link that Bill posted, is the best design I've seen anyone have.
Good luck
Chris
http://s1074.photobucket.com/user/w...mp;_suid=1419452837667028245725198143323http://s1074.photobucket.com/user/w...mp;_suid=1419452851202023412369797683014
