elect to go with the lock nut arrangement the smaller of the two nuts would go on last. IMHO you want a "snug" fit with the key for two reasons. One, you don't want it falling out while your working with your prop under water and secondly, you don't want any movement between the prop and shaft while shifting between forward and reverse which would cause a bronze shaft, prop or both to wear over time. You don't want the key so tight that you have to force anything but a snug fit is what I try for.