Glass does not have to be out to install headliner but it is easier. There are gripper teeth front and rear. With the glass in you just have to tuck the headliner under the gasket.

The front and rear gaskets use a 1/4" round lockstrip.

I have pics of my headliner install (with all the glass, roof rail weatherstrip retainers, and other trim out) here (scroll down): http://70duster440.com/reassembly.html