I have been thinking of doing the same with my 64 Fury. I have a nice original mirror to match the driver side. But I would want to measure the mounting holes to make sure the mirrors matched on both sides. However, I would first have someone hold it in position. if the vent window frame obstructs the view, it may be necessary to move the passenger side one a bit. (I have not actually tried this yet, but I do tend to worry about details.) And, for what it's worth, I have convex spot mirrors on the passenger sides of all my daily drivers to make lane changes easier and safer. I would probably put a 3 or 4 inch stick on spot mirror on the passenger mirror when I install it...
Also, I don't think these are just put on with sheet metal screws. Pretty sure there is a reinforcement to be put inside the door as well.