Who did you go with and
Where would you go for 18" mid or rifle gas with a pinned FSB hopefully F marked and not standard from what I'm reading they are slightly different and may have issues getting zero
16" mid. If you're sold on the pencil and fsb you can replace the sight post with a longer one and make it work.
Longest gas system that's reliable. I think a 20" looks better aesthetically than a 18" on a A2 setup.