I suspect Ram will eventually answer the Ford 7.3 / GM 6.6 with opening up the 6.4 to the full 7.0L. (426ci) And 500 foot pounds of torque seems attainable. Buick made up to 510 TQ from 7.4 (455ci) back in the day.
You might also consider waiting until October when both the new 480 TQ Ecodiesel & 3.0 GM diesels should hit dealerships both supporting new dramatically improved emissions systems that should make current emissions ills largely a thing of the past much like early gasoline cat converters compared to today. Obviously enough power to tow your TT only with dramatically less fuel and a quieter smoother ride.