Since the receiver is the "firearm"(the part that hast the serial #) as far as Federal law goes, once the parts are taken off the receiver, and the receiver cut to spec, you no longer have a firearm. You have a bunch of parts to a firearm, and a cut receiver(now a paper weight) that the parts were once able to bolt, screw, or fasten to. Once imported, the now cut receiver(paper weight) is no longer a firearm as far as the Federal law is concerned, and the left over parts, are well, parts. The "left overs" can then be attatched to a "Home grown" US made receiver, and combined with the proper number of "Compliance parts" (also of US manufacture), and the firearm is thus removed from the idea of it having been imported. It is essentially the same as making the firearm here in the US to begin with... HTH
Still 2 Many Choices!?