Auto manufacturers are continuously making model year changes, so in that regards, it is entirely possible that the frame changed.
When Chrysler and Cummins originally negotiated the deal, Cummins required that the frame be designed by them and not allow the frame design to be dictated by the Chrysler bean counters. It was reported that Cummins was concerned about frame twist. Therefore as others have recommended, do not cut or grind on the frame.
There is no reason why you cannot shim. This is a process used in a wide range of assembly and refurbishment situations.
Shopping for a hitch that "Might" fit without shimming or grinding will require a lot of time and effort without positive results. You will most likely incurr added costs for shipping and restocking.
Are there other trucks of your model year in your area? possibly you could check with their owner(s) so that you can see their frame and hitch brand.
Another option might be to loosen the bolts on the cross tube assembly to see if your frame mount legs will shift out and up tight to the bottom of the frame. Then torque the tube bolts down.