I took a different approach to outboard rearview mirrors. I didn't want to drill holes anywhere on my 1957 MGA roadster's body-kind of like getting a tattoo, no matter if the placement is right or wrong, it's permanent unless you throw out a s---ton of money for erasure work.
I went the way of mirrors attached to the windshield's right and left post. No drilling as they both clamp on via two each hex head set screws.
Benefit: No holes to drill.
Drawback: My aluminum side curtains will no longer fit as the mirror's base plate will interfere, five minutes to remove the mirrors, side curtains fit. Doesn't bother me as I never intend to use my side curtains-car is garaged and never out in inclement weather, top-down California.
No need to go convex, these mirrors are 4" in diameter so between the two and the dash mirror, rear viewing is perfect. They're also offered in 3" diameter but don't go there, too pip squeaky looking as the 4" models look quite natural for the car's size.