Join Date: Nov 2015
Location: California, USA
Cool conversion, this is a long shot but there may be smaller space marine heads out there. I know for my Catachan IG the Infantry Squads and Company Command squads have different head sizes, if I put a Head from the Company Command Sprue on a body from the Infantry Squad sprue they end up looking like Beetlejuice with a shrunken head.
Matching the weapon, throw a Bolt Gun or Sniper or Rocket on them (if available).
If you are using the cloaks in game often then maybe some cloaks as suggested above.
Other than that you can use a few paint tricks to make the body colors pop and the head dull color to help mask the size. Hope this helps or gives some ideas.