Hi Gary,
Sorry, no one here has given you a straight answer. My understanding of this issue is that the rifle needed to be complete by the end of the year 1999 (or at least receipts for everything dated 1999)according to SB23 and in post ban configuration. This is assuming that the receiver is not listed under Roberti-Roos, which could not have been legally purchased after 1989 anyway. Nevertheless, the AG now has the power to expand Roberti-Roos to include receivers made after that law went into effect.
Technically, it is too late because you have not completed your rifle by Dec 31,1999. If you are intent on registering it, you may want to just fill in the barrel length on the form with the length of barrel that you intend on purchasing for it. BUT, if they come looking for receipts your screwed...
Pat.