It should have the same capacity,as everything else is the same. Only difference may be that all 3500's have the HD transfer case,only some 2500's do. Won't do much for your GVWR sticker on the door though. The MTO around here goes by the sticker,not by what is under the truck. It is a costly process to get that number on the sticker changed. I do have some blank stickers from a local truck builder which would do the trick.
I plan on doing the same mod to my truck,when my dump box goes on this spring.
I'm just across Toronto from you,Brampton to be exact,we should get together some time.