Wiring diagrams are available here.
https://www.newtis.info/tisv2/a/en/
The 8 speed transmission uses a different transfer case than the 6 speed. I checked out some ebay photos and the output of the 6 vs 8 speed does not look the same. The best way to do this may be a complete donor vehicle so you can get trans, tc, driveshafts, diffs and mounts. I would love to have the 8 speed in my diesel. E70 LCI diesel in Europe has it.