Factory location is on drivers fender, higher up than the holes in that fender on post #1.
Sometimes people who added radio kits put them on the passenger fender, but the original kit instructions show where to put it on the on the drivers side fender.
I have original instructions from 66, 67 and 69 and all show it there. There is also a couple dimples on the driver side kick panel for where you drill the holes to mount the antenna support bracket and run the wire through.