This belongs in the towing vehicle forum.
The "classic" means it is the same as back to the 2003 models. The 3500 single came in a couple years later, then the mid 2007s changed the game somewhat.
The classic 2500HD and the classic 3500 single, are different by about ISTR 450 lbs carrying weight in the box, but have the same pulling power depending on engine and transmission. The fatter 265 tires than the 245s are heavier on the 3500 and the other heavier components eat up some of the extra GVWR so that rated GVWR difference is more than the actual difference in carrying capacity.
The 2500HD with Diesel can pull more than the gas version, but the box will be way over-loaded. The 3500 only helps with that a little bit.
People do it anyway, so you get all those threads over on the towing vehicle forum where people say it is ok to go way over the truck's ratings and other people saying they are idiots.
It never ends, so have fun over there with this old chestnut about 2500HDs and 5ers. :)