You'll need your grandfather's current registration and title if your have it. Probably a copy of his death certificate. Even though the truck isn't running he'll most likely have to register and insure it, not to drive it but just to make it legally his in the states view. He should do this asap especially if his or his heirs intention is to ever fix it or sell it. Even junking it will be easier if it's legally his in the eyes of the state and that's best accomplished with a registration. Sorry about your loss.