The parts are already US made. So using original parts is OK.
You can apply for a SBR stamp if you want to use the original barrel.
Or, swap it out with a 16" long and save $200
I have not done one of these up myself, but Dutigaf is well within the legal bounds
with his conversion. I'd scrutinize his tutorial and make sure you understand his
proceedure. I think he would be glad to answer questions you may have.