I second the recommendation of contacting ARP. Consider their offerings for either a barrel or complete 6.8 SPC upper. There are few folks around that know more about the platform than H. (Harrison Beane).
I am also a satisfied customer of ARP.
Some good information and vendors here, too.
XD-40 service, XD-9sc, member GeorgiaCarry.Org, National Rifle Association, Gun Owners of America, North American Hunting Club, Rocky Mountain Elk Foundation