If you have an AR-15 Lower already, submit the paperwork (or wait for efile to come back up) and it's $200. If you don't, spend $80 on a stripped lower and submit paperwork (so $280).
Order can at the same time. Don't wait for 41P to come into effect and then decide you wanted another NFA item.
AR-15 lower won't work for his .308 project.
I thought he wanted a 5.56/300BLK one?
I say send off forms for them both at the same time. Build out the lower as a pistol with the Sig Not-a-stock until the stamp comes back.
I bought a 308 can before I owned a 308. Gave me a good excuse to buy a 308 plus you can throw it on a 556 in the interm.