For Mike, here are some rough measurements for stock rearview mirrors (the round kind) on the driver’s door. These measurements were made on a set of spare Terra doors I have socked away in the garage. The holes we’re looking at are the two farthest apart on the outside–the ones on the inside were made after the truck left the factory.
Drawing a line straight down from the rear edge of the butterfly frame, the front hole is 4″ ahead of the line and the rear hole is 1 3/4″ behind. Drawing another line straight through the middle of the two holes, when I put a tape on the bend at the top of the door, the holes are roughly 2 1/4″ below that point.
For an alternate measurement, I started the tape at the very top of the door above the handle and brought it forward. The front hole is at 32 3/4″ and the rear hole is at 27″.
Another good way to double-check these measurements is to pull the glass out of the door and see where the holes come through on the inside; there’s a welded backing plate on the inside of the door skin that should confirm/narrow my measurements.
My Scout (a 1976 of unknown pedigree) features mounts that are completely different. It turns out the backing plate is moved back about 3″ and isn’t as wide as the one on my spare doors, so I can’t mount the OEM mirror I have on the shelf without drilling into non-reinforced doorskin. The original holes in the plate I have measure roughly 3″ apart. There’s no lower plate provision for a western-style mount, or filled holes from a previous install.