I didn't notice your car had a missing mirror. I thought all the later Mantas had both mirrors and it was only optional on the earlier cars (how the hell did people reverse park back then?) but you learn something each day.
As above, I've done a "mirror runner" swap before and it is an easy job. If you get the part, you can see where the bolts go. Plus you could sell on the blank one to someone who wants the Englemann style 400 mirrors (which could also be an option for you, but means blanking off the driver side).
If it's a case of not wanting the bother, like I've done with my Cavalier, you can order a universal clamp-on mirror which won't match and you'd obviously take it off at shows, but makes driving and parking so much easier. This is the one I use. You just clamp it to the gutter and with the mirror bolt loose, then hold the mirror in position as you tighten it. https://www.ebay.co.uk/itm/175388908144