Whew, it is amazing! Saw your clone troopers on artstation and didn't even knew it is a such huge project. Great work and respect, definitely gonna keep an eye on a project.
You can basically just make them square and then resize them before importing, keeping in mind that they'll be stretched in the respective direction whilst you're unwrapping.
Make sure that when you unwrap it, you have the keyboard and such in their respective places, so you can make an LOD by just deleting the keys and the white box and the.