There is a metal support bracket underneath that area of the cowl which you can see from inside of the cab. There are holes in that bracket for the mirror bolts to pass through.
I wrap enough electrical tape around the end of an automatic center punch to fill the hole in the bracket to center the punch, then push on the handle of the punch until does its thing and leaves a pimple in the paint on the outside. I then convert the pimple into a dimple and drill the hole from the outside.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission