Except for Boatycall's response, this thread has more wrong information in it than correct information. It is entirely too common for one response to become gospel on this forum.
The only thing I'm going to comment further on is don't believe a word the salesman or anyone at the dealership says. If their lips are moving, they are lying. They only want your money and will do or say anything to get you to part with it.
DOT has nothing to do with your door jamb sticker. That is a weight with DMV on your title. You cannot change that no matter how many mods you make. So yes you can register your truck with DMV for whatever max weight you want and you'd be "legal" as far as registered weight goes. Having said that, I personally don't play the weight police game, you have to do what's right for you, your ability, and what you know your truck is capable of. If you don't, then maybe you might want to stick with that door jamb sticker information.