Quality custom barrel mfg., 1 in 10 twist, 18", rifle length gas or piston, adjustable gas block and threaded muzzle for flash hider/brake/suppressor. Have the BCG matched to the upper/barrel extension when having the barrel installed for proper head spacing, free float handguard.
I would strongly recommend Vltor A5 receiver extension kit for the .308s on your lower, Geissele FCG.
Make sure the upper mates to the lower. AR10s are not standardized like the AR15s. Not many of the mfg's of the uppers and lowers will not mate up with each other due to proprietary designs and parts.
Spend the money and get the quality parts and do it right the first time. Saves money.