The threaded rod you have your prop nut on is extendable.It is just a piece of rod with a lathe dimple at one end and hex key insert at the other.It is probably lock tightened in place, so double nut it and back it out to desired length.It is best to verify that you will still have a min. of say 1/2" still threaded in the main ,than lock tight it in place again.
Good luck, Bill