There are a number of ways that this can be achieved:
1. Sculpt all of the heads for the army with elaborate iron grotesques.
2. Use the Chaos Space Marine Raptor Champion head for all of your models.
3. Look for another, somewhat compromised, solution.
I don't have the time to really hone the skills to tackle option 1, I'm sure that if I took my time and really practiced day after day I could do it, but I'd rather spend the time on painting more guys. Unless, of course, someone wants to pay me buckets of money to practice sculpting ; )
I have a handful of the Raptor Champion heads, but not enough for a whole army.
That leaves option three. I'm going to use a mix of the ultra-cool Forgeworld Renegades, converted Cadian helmets, and converted Beasts of Chaos Ungor heads. For the latter two I'll carve a line around or ac
ross the face so that I can paint part of the head as an iron mask. I'm confident that this (although a compromise) will allow me to get the idea of the Blood Pact across well, as well as allowing me to get the army done in a reasonable amount of time.
So here are a few of the troopers. The one on the left is on a spare base as he'll be added to the heavy weapon base as the loader. Below that is the rest of the Command Squad.