The receiver extension is incorporated into that stock, so you need the UBR 2 (UBR Gen 2, ) to run an effective recoil system on a .308 AR.
Then, you need the Armalite EA1095 recoil spring, and a 3.250" long H3 buffer. That's either any old AR15 H3 buffer, OR the Armalite AR-10 Carbine Buffer - those are the same exact thing.
Ask questions about the info, if you want, but that info is solid.
Lastly - you just received Free Information. Do what Doc said, and hit the intro section, and tell us about yourself. Pay for that information, by doing that.