I thought the minimum Federal length was 18 inches for rifles and shotguns. This is the total barrel length which, with a cartridge gun, (egads!) includes the chamber.
As for the old originals, if they were MADE with barrels shorter than 18 inches BY THE FACTORY prior to 1899 (or 1898, I forget) they fall under the collectors antique area of the law and are outside the jurisdiction of the Federal requirements.
If I were making a recreation of a muzzleloading double barrel shotgun for stagecoach protection I would use 18 3/4 inches just to be on the safe side. I would also remove any and all traces of it's being sawed off by filing, sanding and reblueing or rebrowning its muzzle.