Two ways of converting a helmet from oxygen mask to boommike. (All pics are thumbnails)
Left picture shows modified CX-4707C/AIC comm cord where the inner cords have been resoldered in the U-174/U plug as follows: Green cord from 1 to 4, red cord from 3 to 2, white cord from 2 to 1, and black cord from 4 to 3
Middle picture shows a modified headset where the standard CX-4708/AIC comm cord has been substituted by a comm cord for an SPH-4 helmet. Two holes have been drilled in the S-hook to allow fixing it to the helmet shell.
Right picture shows the modified headset installed on an HGU-26/P helmet. Note that the mike cord has been fixed to the helmet shell using a nylon clamp and that the S-hook is fixed with two screws.

And if you prefer not to mount the boommike swivel directly on the helmet shell, you can also mount it on a modified T-bayonet like this.