As others have said it sounds like you need to lock out 6th to keep your RPM's somewhere around 1900-2100. That is the sweet spot for the Cummins and it will pull like a train.
My combo is great in that I sit right in that range in 6th gear right around 65 mph so I can actually pull my 16k fiver up some pretty good grades without downshifting and it will actually accelerate up the hill if I give it some more throttle. Trust me, your new truck has MUCH more pulling power than your old 7.3L, it will just take getting to know it better to get it out of it.
Please let us know how you make out!
