Looking at the Keystone website the 38XX Montana GVWRs are in the 16XXX range. That means a possible 2800-3200 lb pin weight.
The '14 Ram Body builders web show some 3500 SRW crew cab longhorn/etc 6.7 Cummins has payloads up to 4500 lbs. With the right truck selection the 3500 SRW will handle a 38xx Montana.
The 3500 DRW 6.7 Cummins Longhorn/etc crew cab long bed has up to 5800-6000 lb payloads.
The standing with your feet together or apart is a poor analogy at best of the difference in the new gen SRW vs DRW.
The SRW with those big 18" or 20" E tires at 3700-3800 lbs capacity with a 9"-9.2" wide tire tread/wheels aren't the old skinny LT235/85-16 E tire with a 7" wide tread from past years.