You would be manufacturing a Short Barrelled Rifle (SBR). Go to the ATF websight and you can read all the rules for a manufacturing license. You would probably be much better off financially to get the $200 tax stamp, with your local CLEO approval, and just buy a "D" model. Look at the KT site, it's probably just what you want.
As redneckrepairs mentioned, without the proper manufacturing license and/or tax stamp for owning, it's worth many years in a Federal Pen.. Similar to what would happen if you put an AR pistol upper on a rifle lower, you get to be someones girlfriend for 10-20.