Sounds like a 90 day permit to me from the purchase State, then register it legally when you get a real address. Why the LLC other than tax ummmm.... Avoidance? So what business are you going to have in Montana?
That would have to be one hell of a tax bill you are worried about to even consider it. Montana and SD are cracking down on those LLC's. It's not the police that should be your concern. Don't you have to have a physical address to get license and insurance on it before you leave your State?