The Gen5 trucks use a different communication architecture than the Gen4 trucks, this could be a limiting factor and/or make the swap really expensive.
With the weight of the Cummins and amount of torque possible, you're probably looking at a frame swap if you plan on towing or hauling with it. The EcoDiesel is really close to the 5.7 Hemi weighs, it's the exhaust system that adds the extra pounds. A Cummins is a boat anchor, it's about 800 lbs more than a Hemi
Your going to want to find out before hand if someone is able to tune/build an 8HP90 to deal with therpm/ torque curve that a Cummins will throw at it.
If you do end up using an 8 spd, probably no reason to use a lower axle ratio. I have the 3.92 (w/LSD) with the EcoDiesel and in wet or slick weather the truck breaks traction really easily (tires are three peak rated). Remember that diesel torque is all down low, part of the reason for using lower gears on a gasser is to get the engine to the much higher torque band rpm.
