I've always thought this half-helmeted conversion was a cool look. It really only worked with Grey Knight Terminators though due to the way their helmets were constructed. This one was done to a metal model which was tough to do, but now that the new plastics are out, this conversion should be a piece of cake to pull off.
The idea originally came from the back of the old Daemonhunters codex. There's a model done by John Blanche that has this conversion done to it. I swore I would do it to one of my models one day if I had the chance. When I had the opportunity to build a few Grey Knight terminators a while back, I knew I had to try this out.
Using the old metal models made it tough to do, but not impossible. Some careful filing and test fitting until I had both parts married together. The new plastic pieces should make the fitting much easier.
The key is cutting away enough of the head
You need to cut a way a good portion of the head and leave really only the nose upward showing. Even then, you cut off the bottom of the nose as well. You want this guy to be peering over the bottom portion of his face mask.
The lower portion of his helmet is much higher than a normal collarpiece would be and covers a good bit more of his face. Make sure to leave the back helmet vents in place as they frame out the top of the head nicely and give the illusion that the head is set down in there. You may have to cut away a small portion of the back of the head to get it to fit on there, but that should be no problem.
And you can do this with just about any bare head out there. Having hair on there might be a bit tough to pull off, but any bare head should work just fine.
Make sure to check out these posts as they might help:
How to paint power weapon lightning effects
How to build a Terminator Chaplain skull helmet