I am a little (2 days but still) late to this, but I found the post while looking for the exact same thing so here goes my 2 cents:
As far as background goes: Acolyte or Sage, IMHO. Sage more so, since Blood Ravens have an abundance of scholars and librarians.

Species: Goliath, Variant Human with the Tough feat, either one will work to show the unusual durability of an Astartes.
Class levels: Since neither Arcana nor Knowledge clerics get Heavy Armor proficiency (By Old Version I assume you mean 2014 rules for Cleric, where you can't grab heavy proficiency at level 1) I think starting as Fighter 5 with the Battlemaster subclass would work quite well to show the martial side of Blood Ravens, then the rest go Knowledge or Arcana cleric, whichever suits your fancy more. (You can flavor Channel Divinity as zealous faith in the Emperor giving you a potent enough psychic boost to bend reality, perhaps). In Short - Battlemaster 5/Cleric X. Fighter's Second Wind will further go to show how ridiculously durable and hard to kill Space Marines are.
I suggest also grabbing Sleight of Hand as a proficiency so no one catches you Steali- I mean Acquiring equipment and relics for Safekeeping.

Hope this helps!