I did put on my X quick hitch (X485 with the mower deck in place). It was a tight fit. However, the X series hitch is different from the 400 series hitch (the 400 will go on the X, but the X will not go on the 400). So, I don't think the old 400 hitch will go on the X series at the same time the deck is on.
When I put my X hitch on my X485, it contacted the threaded ends of the mower bail, but wiggled by. If the threaded rods ends were cut back (they're at least 1" too long) then I think there would be no conflict for the X hitch to go on the X series with the mower deck on at the same time.