i'd have a gunsmith do the cutting and other associated work (not intending to insult anyone here in any way). the SBSs that i've seen, handled, used, etc. were generally much better off when they had some professional help. one model created by a friend and gunsmith named Richard is called the Wristbreaker. its been somewhat featured to be more than just a hacksaw job. enlarged and widened trigger guard, a new stock (well, actually its a pistol grip), new forearm, new break lever, break lever guard, etc.
just my .02
as to your original question, you can do the paperwork and manufacture the associated barrel (in this case, manufacture means saw off). i'm fuzzy on SBSs, since i don't own any or have much interest, i highly recommend a careful read of the requirements, and perhaps consult with some NFA/Class III people. i may be preaching to the choir here, but in MD, i know a lot of people into NFA stuff, and we are all doubly careful to keep our tracks well covered legally.