I would definitely go with a quality milspec receiver extension and BCM does theirs right. I'd upgrade to a heavier buffer too, depending on your upper. PSA has good deals if you watch for them. Anyway, any good carbine stock I can think of off the cuff is available in milspec diameter.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek