Re: registered pistol lower...
AFAIK once a gun is a pistol,or Handgun,from a factory,it can be turned into a rifle.But,you cannot turn a Rifle into a Handgun.I'm going through something similar,I want to build a REM 870 into an AOW,as a 10" bbl breecher,but I need to find an 870 that came from the factory w/o a stock,just a pistol grip and be able to prove that.
It might be easier for you unless it is some really special Rec. to just buy a nice one and start from scratch especially when you are dealing with an NFA item.Big enough PIA even when your paperwork is in order and nothing unusual is involved.