Welcome to the board.
You can fit the topcase but you have to modify the mounting plate.
I did the same thing back in April, it didn't take me long at all, I just used a drill and some hand tools.
There are several threads on this.
Here's probably the best link, a member detailed how he mounted his, with pics:http://picasaweb.google.com/chuckdd/JCWCaseInstall
Here are two more threads that may help:http://11109.rapidforum.com/topic=108770128619&search=whitneyhttp://11109.rapidforum.com/topic=108771016319&search=whitney
Good luck and enjoy the bike.