The cab is exactly the same (i.e. doors, outer cowls, inner to outer cowls, all inside sheet metal...). Basicly from the firewall to the front is different, meaning bigger because of the bigger wheels/tires. The outer fenders, inner fenders, bumper and hood is larger. The (5) horizontal grill bars and aprons are the same for all. The vertical bars for the grill are different, again bigger, due to the larger fenders.
Vendors do no carry larger sheet metal (i.e. hood, fenders, bumpers...) but they do carry perty much everything you need for the cab. If you need the larger metal, do can post it in the "Truck Parts Wanted" section. You just may score on what you need! I did, on everything I needed for my 6400 so far.
Hoped this helped.
