I have both the fast gun and the dual for my classic! What Hans said about comfort; ditto! Just did 2700 miles in 6 days on the fast gun, not once was my backside sore!!
Also, what warp said about the factory is accurate! Give them a call; chances are it's NOT too late!
As for the bracket, I love that my fast gun fits right over it with the normal locking system. No need to remove! I welded thumbwings onto the bolts to secure the double...literally no time at all to swap