For the 2013 2500HD and 3500HD --
rear overhang for standard box is 47.6"
and rear overhang for long box is 52.5".
Combinations of Wheelbase and Rear Overhang are listed below along with the tongue weight required to add a load of 1400# to rear axle when no WD is applied.
Column 1) Wheelbase
Column 2) Rear Overhang
Column 3) Tongue weight required to add 1400# to rear axle assuming Ball Overhang is equal to Rear Overhang plus 10".
133.7" 52.5" 954# 4x4 Regular Cab Long Box
144.2" 47.6" 1001# 4x4 Extended Cab Standard Box
158.2" 52.5" 1004# 4x4 Extended Cab Long Box
153.7" 47.6" 1018# 4x4 Crew Cab Standard Box
167.7" 52.5" 1020# 4x4 Crew Cab Long Box
I see nothing in the above numbers to suggest why there is a 500# difference in maximum allowable tongue weight.
Ron