Factory visor instructions can be found here:
http://chevy.oldcarmanualproject.com...cpctai206.html
The original visor mounted under the edge of the cab above the glass using sheet metal screws and lockwashers. The original instructions call for a 3/32" drill. Aftermarket visors may use different fastening methods and I'd advise looking at the instructions for the visor you are installing for specific information.
I like nutserts. We often use them to install signs on the rooftops of small vans and they stay put for years. I would probably consider using them for a visor installation.