I haven't tried 7.62x39 mags with 6.8 but I know the 223 mags don't hold 6.8 well. You need a different follower and if you swap out followers on the LAR mag I think you may be making it illegal. Not sure if you can modify it and have it remain a legal pistol mag.
I don't really see much point in trying for higher capacity in the 6.8, it's too expensive to be popping off rounds rapid fire just wasting ammo and I plan to hunt with mine so am limited to 5 rounds for that anyway. I would swap back to x39 or 223 for when you want to shoot higher volume.