You can't double OD with these units for mileage gains on diesel manual trans trucks. GV says so right in the application charts.
I don't give two ***** about keeping the trans cool running in 4th with the GV unit instead of running 5th, especially when I only pull 6-7K, not 13.5 like my rig can pull. My trans isn't going to get hurt doing that and I bet I could fab up a cooler system for a lot less than 3K!
No fuel mileage benefit, no thanks!
It may bolt up OK, you still need a shortened driveshaft.