I made it across the country. A few weeks ago. Goats and all. I managed to convince a friend to take a vacation and come for the drive. I have 15 gears, my turbo works awesome. I can pull sustained grades at appropriate highway speeds, many times even able to accelerate and pass up big rigs easily. My fuel economy in high elevation mountain regions with lots of big hills to climb is about 9.5 mpg. In Utah, Nebraska etc, I average about 12. I am able, if I choose to accelerate all the way to 90 mph. I can cruise 70 mph. When I am in double overdrive I cruise 70 at about 2200 rpm. I occasionally rev all the way to 3000 rpm. With no ill effect. My exhaust gas temperatures and coolant temperature never entered any dangerous range even while climbing the Rockies on a hot hot day loaded down with 2k pounds of cargo. A hail storm in Nebraska ( no kidding ) took out my radiator, so I spent the night on the side of the road waiting for a new one. So now I installed a gold screen to protect the new one, very stylish haha. The goats loved the trip. They rode inside. I decided to forego the up top concept for the time being. I have yet to adjust my IP. Even so with it at stock setting, I build about 7-8 psi at2600 rpm, with stable egts of 1000 degrees on a hard pull. By adjusting my ip I should be able to safely achieve about 10-11 psi peak boost without raising my egts or over stressing anything.
So here are the bullet points.
Yes you can turbo a 6.9
Manual transmissions are more fun.
Two transmissions are even more fun
Never tell Me what I can't do
I'm getting 200 some hp with 12mpg in 13.5k lbs bus
My goats loved the trip
My dog loved the trip
My bus goes faster than yours. ;)
Ps. I just took a trip to the coast in her and let me tell you, she carved highway 140 like a pickup truck.