If/when you have to do body work in those areas, you could fill the holes then.
If you want to go to original mirror-arms/mirrors, drilling the holes are a contortionist's bad-dream, but where you drill each pair of holes is, luckily, not hard to determine. Look up from under the dash (on each side) and you will see a "brace" on each side, directly under where the each mirror gets attached. I think the hole-drilling locations are fairly apparent from underneath?
Pictures have been posted in old post, but the links might be bad.
