Let me rephrase. The op asked if he could run a Six8 upper on a normal LWRC lower. Why pay extra for a part to not fit correctly and not be able to run the 6.8 Pmags? Just get a regular lower.
The idea behind the Six8 is to run the pmags. Only this lower will take the pmags and you have to have the upper as well.
If you’re going to mix and match you can’t get the benefits of the Six8 receivers. So just buy regular mil-spec and save $400.