Driver's mirror

Unfortunately it's an 'it depends' ... my understanding is that mirrors were typically added at the dealership as part of the prep, so there wasn't a lot of consistency in terms of the type of mirror or how they were positioned. Some received just a mirror on the driver's side, some both sides, some were way forward on the fender, some got flat chrome Mustang mirrors, others bullet shaped mirrors ... I think a lot of it depended on what was in the dealership parts inventory - I was at a show this weekend with 5 Panteras - 5 Panteras, 5 different mirrors all mounted differently.

That being said, are you able to remove the inside door panel and see where they were mounted in the past? Another option is to grab a steel or heavy plastic plate or even some washers and mount the mirror where it looks best to you and gives you the best visibility.
Here are some shots of other cars if it helps ...

Cougar Sport (I think)

Fender mounted ... pretty sure these are a modern addition

#5754;
Thanks for the info. Mine looks very similar to the one in the first shot with your post. Probably an early '70-'72 era from Cougar stock. Appreciate the information.