We don't have that rule that you need a CDL for a truck with a gvw higher than 26,000lbs a regular DL can be used for any 2 axle truck with any gvw. The highest rated single axle is 35,000lb if you got a regular DL you can drive it but most likely you would need a air endorsement.
The Cummins can't get that much better mileage over the PSD then again Dodge trucks are lighter than a equivilant Ford so you may gain something there. Myself I would rather own a MDT truck like a T-300 Kenworth for doing the job you have you got more pulling power and probably get the same mpg.
Myself I have a CDL I'am gonna upgrade to a class 01 (A) right now I can only drive a tandem axle straight truck or pull anysized trailer combination if truck and trailer is hydraulic brakes. Upgrading to a class A is worth it a guy gets more work plus in the work I'am in you gotta beable to pull pup and tag trailers behind gravel trucks.