As it turns out I wasn't quite done. After getting everything together I realized the head needed to move up and back, which meant I had to machine a new neck pivot and then adjust the control rod length. Here's the new neck pivot assembly.
The really fun part was the caps for the fan mechanisms- once I hollowed them out they didn't fit- doh! I thought I took really careful measurements but close wasn't close enough so I had to make new linkage parts so I could get the caps to fit properly. I also had to add the the little brass sliding tabs that cover the open gap as the fans rotate- just like on the original movie helmets.
Now the caps fit properly. They're held in place with magnets and a round socket that fits over the fan mech pivot retaining collar so they can be easily removed if any repairs should ever need to be done.