If you don't offroad it you should be able to fit 35s. They might rub if you flex it out though. They shouldn't void the warranty for that. They can only void a warranty if they can proove that the aftermareket item caused the problem in question. So obviously there won't be any tire warranty from them, but that should be it. If you get a real a$$hole service writer they may try to blame the tires for trans or axle problems but it's unlikely.
As far as the speedo, I dunno how it is with the new style trucks but for my 2000 I just got a Superlift Truespeed for like $120.
As far as the speedo, I dunno how it is with the new style trucks but for my 2000 I just got a Superlift Truespeed for like $120.