You're moving right along, from parts build to complete upper! :p
I read your OP and didn't know enough about 6mm ARC to post a suggestion. What I do know is that it was designed around an 18 inch barrel for the .mil's multipurpose combat rifle program.
I believe you can find more 6.5 Grendel barrels than you can 6mm ARC right now. I have a LaRue 6.5 Grendel and it is a dinger - I generally shoot at 10" plates on the back berm of our rifle range so about 620 yards, it just thunks them on in there so I'm running less than 2 MOA with Hornady 123gr ELD's - unfortunately they no longer list that barrel or the Ultimate Upper kits.
This thread on Sniper's Hide will give you a headache with the comparisons:
https://www.snipershide.com/shooting...-2024.7202648/