[lofly'a]

"for best results", that term is important sometimes, use the factory mounting method. i assume that your truck is the 67/68 in your avatar so the the factory mirror components will look like the pics below. the backing plate is the threaded mounting point and goes inside the door. drilling and tapping the door metal IMO is not the way to go (not thick enough), especially if filler is in this area. so from inside to outside you will have the backing plate-door-gasket-mirror, this is the sandwich called a mirror assembly that is all held together with 3 screws. most of our vendors have these pieces from $40 to $50 each. even if you use another type of mirror, use or fabricate a backing plate, or you'll be tightening or reinstalling mirrors often