Looking at the pdo of both parts together, it dosen't look like they were modeled at the same time. The attachment dosen't match up with the contour of the helmet. Here's a pic of what I mean...
Short of modifying the 3D model itself, it looks like your option is to build the attachment (ensuring the scale is the same as the helmet), resin and harden it, then do some major adjusting to get it to fit.
Your helmet looks pretty good. I can't wait to see that bad boy finished.