There is a picture of where to drill the holes and what size in the 60-66 factory assembly instruction manual. The holes are 21/64" in diameter and 10.10" apart. The spear gets one clip and the long piece gets five for a total of six holes. The elivation is a little more difficult to explain. The best thing to do would be to line and level it off the piece already mounted to the door. Place one clip in the middle of the spear and hold in place come back toward the door 10.10" to drill the other five holes. Or you could just get the manual, a picture is worth a thousand words.

My 1947.1 Chevy 2-Ton Flatbed