Yes, the lower is the part that requires registration and taxes in the case of th M16. The AR15 lower is simply a rifle receiver, no special requirements beyond that of any .22 cal rifle receiver. You may possess any and all M16 parts and they are all perfectly legal, whether or not you have an AR15 lower. The M16 parts will not fit into an unaltered AR15 receiver. What you may NOT do is machine out the AR15 lower to accept the fully automatic M16 parts. That constitutes intent and constructive possession in the case of possession of all the parts.
And the manufacture and private possession of any new machine guns was halted with the 1986 change in the laws so there's no way you can legally make an M16 out of an AR15.