Well, making the assumption that the 605s have the same thread pattern as the 764s, but are either shorter or longer - you may or may not have done damage to your engine yet, but pull them out immediately and get the other set of 764s. If the threads were different and you forced them in, you'll have to look into retapping the threads in the head or replacing the whole head. How the hell did you manage to get a different package of plugs and not realize it until you had already put them in??