I believe all the holes for the Air-Flow and recirculating types are pre-drilled or punched. NOS heaters come with directions and templates for installation as well, if a hole is needed they give exact dimensions.
https://www.flickr.com/photos/advance-design_parts_co/14237974312/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/8129418873/in/album-72157631868452754/