My last RAM 11 HO Dually 68RFE trans with 3.42's towing a MS combined weight of 29K. I towed in 5th the same rpm's as if it were in 6th with 4.10's. Truck pulled the weight VERY well. Only issue was getting the load rolling on a grade. Now that the Aisin is available with it's lower starting gears I would highly recommend the 3.42's with Aisin up to 27-28K and anything over that 4.10's. 1,750 at 60 the speed I tow at is right about the Cummins peak torque rating.
My combined weight is 32,500# and the 4.10's at that weight are for sure the way to go!
Based on my experience I would get the Aisin no matter what gearing and I would either get the 3.42's or 4.10's and forget the 3.73's.