What is your model and serial #?
By SD do you mean FasTrak Super Duty 60"? or 60" side discharge on a Hustler Z or Super Z?
If its the Z or Super Z the blade lengths changed in 2006 with the introduction of the XR-7 decks.
This may be your problem.
Let me know what you have and I will make sure you have the right blade part #'s.