5.56 to 6.8 On A Mono-rail

I have a Colt LE6940 and LT6720-R TALO (I like mono rails). I'm thinking of converting one of them to 6.8 by putting in a new barrel and bolt. I have read in a few places that the gas blocks on mono rails make this difficult but not impossible. Aside from that, which one would be the better choice for conversion?

thanks