If you're going to stretch it, I'd stick with getting a whole new swingarm instead of doing extensions, unless you want to be able to change back to stock wheelbase "on the fly" (which is really not on the fly at all... but less time consuming at least). It looks better than the extensions.

Why are you stretching? Drag racing, or just because?

If you're stretching, I would also suggest lowering links. If you're racing, I might also suggest a strap for the front end.