There is a lot of over-thinking going on in this thread.
Osprey21 is right-on. The only thing you should do is replace the selector with a semi-auto AR safety. I did not even bother pulling out the sears when I did ours.
You should not use those stupid selector blockers. They snag other objects, are not finger friendly (they shift the pistol grip as well), and they break. Sometimes people make them break on purpose. That doesn't happen with a safety, and they are inexpensive. They're also easier to swap. Usually you can get away without having to completely remove the pistol grip, have the selector at about 45 degrees and knock it out, then hold the detent down when you slip in the new safety. Simple.
If you insist on using the stupid NSN block, I believe there is/was a place to get them for free (or close to it) within the government. Don't make me dig for that old link though, because they're dumb... and stupid. Did I mention they were dumb and stupid?