I'm in the process of putting clip ons (aftermarket) on my XB12SS, but I was originally planning to use the Firebolt upper clamp and clip ons. If you google around , there are a couple guides on how to do it. Essentially, the only "trouble" areas on the Firebolt clamp are that you have to grind off the ignition loop, and you need to fab up a small bracket to bolt to the firebolt clamp since the lightning gauges etc... bolt to the upper clamp.
Other than that, you MAY need to use shorter clutch cable/ brake line depending on your routing. And I've seen some people say you need a smaller master cylinder to clear the flyscreen area (or just space it out).