I'll have to watch ALL the video later. But I'll make some comments:
- His truck is too quiet.
- I didn't watch but maybe a 4th of the video, so maybe he states this, but I'm guessing he'll have to drop it out of 5th over, to 5th original when pulling steeper hills on the interstate.
- I'm betting (whether he admits it or not), he doesn't get much use out of 5th over when towing heavy.
I know those trucks well. His sounds more stock than mine. And especially with the stock air box and a muffler (and maybe even a catalyst), they just don't make a lot of boost below about 1,900 rpm. He might get good mpg empty cruising the interstate. And he might get good mpg going east in Kansas (meaning flat ground, no headwind) when towing heavy. But that GV O/D will mostly be annoying towing heavy on the interstate, not enough rpm to make good boost in that truck.
That GV unit is not a bad thing. Especially on 6BT's with a 5-speed. Mine is a 6-speed. But they are 'spensive. I don't think you'd ever save enough fuel money to pay for it.