you should be able to, however feeding may or may not be optimal or issue. I do not have a 6.8 spc build nor have I researched how critcal M4 ramps are to that round. It wont be optimal because youll be missing some guide surface of the upper M4 cuts.
The only combo that i know to stay away from is the....Upper with M4 feed ramp cuts and a barrel extension with no cuts, as this leads to a ledge that will catch rounds.
BTW wrong forum, this is in the AK group build subforum vs the rifle forum