That top screw can be a booger to get out sometime.
Once the guide is in place the channel slips down in it and the end of the channel slips down and behind the "clip" on the guide.
...be sure you have the right channel and guide. I believe that on the earlier years there is a width difference. Good luck and let us know how you make out.
Online chevy shop/assembly manuals here. Take your time and find what you need.
http://chevy.oldcarmanualproject.com/
