There is way more involved than just an extra leaf spring...i cant believe you would hang your hat on what a car salesmen told you one time.
3500 QC SWB is equiped with cummins standard. Both auto and man have extra spring and a AA11.5 rear axle 3.73/4.10
2500 QC SWB G56 manual comes with AA11.5 and one less spring. Optional cummins.
2500 QC SWB 48RE auto comes with AA10.5 and one less spring. Optional cummins.
Think back to before 2002... same difference as Dana 70 vs Dana 80
Also, the 4X4 3500 has bigger front springs than the 2500 kid brother.
It also depends on the wheel base...long bed, short bed, single cab or megacab
My 2007 3500 2wd 5.9 G56 megacab dually has one less leaf spring than my 2006 3500 QC 4X4 SWB. They have the same axle, same motor, same gear ratio.