the dual kits leave loads of spare heads, depending on the build...
anyone at your local shop that has built Infiltrators should have a spare Ruststalker head for you...
the Green Stuff route is easy enough, too, since you don't have to try and get into the cowl to sculpt a few flat plates...
hell, even a little creative use of cardboard from a cereal box could do the trick, especially if you slice the nose off of the face, and flatten out the brow and cheek first...
the more you model, the more you learn little tricks to make life easier
cheers
jah