i have a 5" spacer on my engine to make it 25" now. i dont know where it came from. i did a google search for you and came up with this.
http://www.marineparts.com/PARTSPAGE...ories/ACC8.HTM
or google search for "outboard shaft extension"
i cant imagine it being any harder than a water pump, when i just did mine on friday the spacer comes off with the lower unit.