YOU have to make at least the frame.
YOU are dead wrong. When you file a form 1 on an existing firearm you become the NEW manufacturer. YOU must engrave YOUR NAME and CITY and STATE on the existing receiver. In characters at least 1/16" high and .003" in depth.
All caps is not required.
There are a number of importers bringing in 1887 clones that are more than suitable for making into a SBS.
After YOU have YOUR approved form 1 in hand and YOU have engraved YOUR information on the firearm, YOU can even have someone else work on the firearm. It is important that you be there the entire time the work is being done. This is because the firearm is now your NFA controlled property and if you left it with a gunsmith that is not a 02 SOT (manufacturer) you would have illegally transferred the gun. Do YOU understand?