how do they get the extra 700lb load capasity on the 3500srw vs the 2500hd?

I'd suggest doing a search. This has been talked about alot into great detail. :cool:

bigger leaf springs,and all steel rims,with e rated bigger tires......in a nut shell:D

They make them out of better steel. :D

They actually don't have bigger leaf springs, they have more of them in the rear. Some of them have overloads, and others, like my 06 have two more leaves back there. Also, they have 265/75R16 LRE tires on 7 inch wide wheels standard, that gives you 3400lbs/tire load capacity, vs the 3,042lbs/tire on the 3/4 ton's 245/75R16 LRE tires. I think that is about it. There might be heavier Torsion bars on the front. I know mine sits really high in front.

bigger leaf springs,and all steel rims,with e rated bigger tires......in a nut shell:D

x2
not necessarily "bigger springs" but a stronger spring pack

A buddy of mine ordered a 3500 SRW cc/lb and the dealer screwed up and ordered a 2500HD they changed out the spring packs and ordered a lift for a 3500. problem solved

bigger leaf springs,and all steel rims,with e rated bigger tires......in a nut shell:D
yup

I might want to do the same to my 2006 2500hd. I understand the spring swap, but what's the "lift"?

A suspension lift kit. Not neccessary for this swap.