Need help with Combine Juggernaut

Here’s my idea:

A Combine and a Rebel Juggernaut.

How i’m going to make it?

I Have a model pack full of tactical vests and Military stuff; gonna “glue” them with 3ds Max on the reskined versions of the desired models, and make a head hack to give a rebel’s head to a combine body.
More or less like this (but edited not edited on paint, of course duh):

The thing is, how do i do that?

[editline]20th July 2015[/editline]

That is, how do swap heads in 3ds max?