SkinnyGrey, I think you are reading posts from users who haven't read the whole thread. If the upper and lower are being sent separately to your FFL, and the FFL will pin and weld a muzzle device to bring the length of the barrel over 16" before you pick it up, you are just fine. If you really want some insurance to CYA and prove that you aren't trying to break the law, just get your FFL to print a work order stating that he will pin/weld the muzzle device.
It sounds like you have everything figured out, but the way your first post is worded makes it sound like you are unsure of the law, have mistakenly ordered something you shouldn't have, and are now trying to find a way to make it right after the rifle is already in your possession.