To do all the things you've talked about, highway speeds, pulling a 5th wheel trailer and all. Have you thought about moving your sheet metal onto a modern GM Medium Duty Truck chassis? You'd have to widen the front clip to cover the wider axle, but the complete drivetrain would already be engineered. And to try and fit a Cummins with an Allison into your existing chassis your already going to be doing a lot of custom sheet metal work to make it all fit...just a thought.
Mike B
