See the directions here:
http://chevy.oldcarmanualproject.com/accessories/1951am/51aim65.htm
There are existing holes in the door for this installation.
You can see them clearly in this picture on the square raised area below the oblong hole.
https://www.flickr.com/photos/advance-design_parts_co/8130539326/in/set-72157631870734114/lightbox/