Other than suggesting the Allison trans I'm of no further use in this discussion...putting a 600 HP engine in a AD Dually is not something I've ever thought about doing and I have no idea if there's even an Allison that would be up for that much HP.
I had a 1977 Kenworth W900A with a Cummins NTA-400 and a 13-speed RR with OD and 3.90 rears that would run 85 mph, but your front axle would be flattened if that engine and trans were set on top of it.
The 6100 has a 137" wheel base while the 6400 is 161".
Keep us posted with your project, I'd like to hear more about it!
Mike B
