There is someone on Ebay that has been selling a CD on what they did to swap a Duramax into a Pre-88. Lonnie also has a thread on here somewhere. It is already a nice looking truck, and it isn't done.
If you find a donor truck, I would use the whole tranny and xfer case. Then source a Ford :( D60:D. They had the high pinon. That will help w/driveline agnles.