I've built a 6.5CM off an SLR set. Nothing wrong with them other than all the CNC billet stuff is pretty unnecessary. I would strongly suggest an LMT with the 15.3" mono upper or buying a used SP-10 and building off that so you can get a monolithic or semi-monolithic upper. But if you're set on SLR, it'll work.
These days it's really hard for me to buy a non-custom large frame precision oriented AR barrel. If I wasn't buying a blank and having someone I trust turning the barrel I would have to default to Proof just because they have the gas systems figured out if nothing else. I have one RTR barrel and it would seem like Craddock has as well so given the two choices I would do the RTR. And I'd probably do an intermediate length. No way in hell a mid length for a high pressure cartridge in 16".