As far as the Feds are concerned, to make a pistol, you can use any lower so long as it was never part of a complete rifle.
Get a new lower. The dealer should identify it as 'other' on the 4473. You can then proceed toward making an AR pistol with your upper so long as you comply with state and local laws (which, in your case, I'm unfamiliar with).
Once major advantage of an AR pistol is that you don't have to report taking it across state lines like you do an SBR.
