I just drove the turbo 3208 around today. Fast, but the 5 speed I put behind it is a disservice to the engine. I am moving up to a 10 speed soon.... well, I need to purchase the tranny yet, but it's all lined up. I need some bigger tires to fill up the space between the rear tandems, and I'll be cruising solidly at 70 going about 2300 or so, I mean, that's back of the envelope, just guessing at a top speed. Once I do all that stuff, I will have a reliable highway cruiser, but not until that stuff happens. I've probably spent, say, $4000 on the deuce already, and I'm close to dropping another $2500 on tires and this 10 speed. All in all, for about $10k investment on a truck that does a poorer and more uncomfortable job of running down the road than your random medium duty truck. But, I love it because it's 6x6, which I occasionally use, it has the uber low range for hauling stuff up and down STEEP hills, and it just plain is cool.
Seriously, what's your end goal? If it is to try to shoehorn a big ass, heavy engine into a deuce, along with a big huge 30"+ tranny, you are going to be really REALLY pressed for space. Already, my exhaust is a hair away from the firewall, and I am running a 3208.
If you want alot of power with the cool factor of a deuce, my opinion is to work on a pretty hot rodded smallish diesel, along with a 10 speed or baby 13 or something, but keep the OAL down to a minimim, along with the weight.
Put a tape on some of the components you want to fit in there. A 3408 just sounds pretty huge.
Hope this helps
Bob