I'd think installing the whole swingarm and wheel would be the least work, rather than fitting the 9b wheel to your 6f swingarm.
If the swingarm of the 9 is wider than the frame rails of the 6f, how much material would you have to remove to get one to clear the other? If it totals up to an inch, I'd say it's most likely a no-go. Shaving off 1/2 " on either side would probably cut all the way through the metal of the frame, and I would expect swingarm would be much weaker by removal of that much material. Even if you were to expand the gap between frame rails, and also pinch in the nose of the swingarm... if you did all of that equally, you'd still have to take 1/4" off each part.
If that whole setup is a Bad Idea, then you should be able to source wheel bearings that has a smaller inner diameter, to match your 6f axle. I would be leery of using a spacer to make up the difference -- that would have to be press fitted in, to try and create some stability, and I don't think the odds are good of that.
