One method would be a center section from a 60-72 truck with the GM/Eaton axle. They typically have the 4.11 gear, you may luck out and find a 3.90.
Another option is a complete rearend swap. I don't know if the truck in question is a dually or single wheel, or even if axle width is an issue, but here it goes. GM has a full float 63" WMS-WMS (wheel mounting surface) and a 67" 14 bolt rearend. The good thing about them is that they are fairly common and they were made from 72 to present day, with only minor changes and a pretty good selection of gear ratios. Rearends from Dodge and Ford vans seem to be another popular swap. The Dana in the Ford is kind of hard to come by. They mostly were built with the Sterling axle, which is still a possibilty. You'll want to look for a full float Dana axle in the Dodge vans otherwise it'll probably be the Dodge semi float 9.25 rear.
