Hi Andrew. Thanks for your comment here. We just got our first batch of prototypes in for the heads and guns. I'll be doing a lot of painting and gluing over the weekend for pictures, but the short answer is that they do fit in the head socket as intended.
We're currently working on a torso you might be interested in as well. The end goal is to create enough conversion parts for you to make an entire space marine with whatever weapon(s) you need from our bits should you desire to do so. So please keep an eye out in the coming weeks.