Weren’t these dealer installed in a place they liked them? 
Sure, there was a template offered, wasn’t there? I did a Search on this site and came up empty.
Maybe the template was for rear antennas…
Oh, this moldy brain…
Have an assistant sit in the drivers seat and tell you when the passenger side mirror is in the optimum placement. I did it alone-drilled the two holes and guess what? Darned if the rear hole ain’t alittle too far outboard. Another hole in the wrong place to be welded up at next repaint, along with the holes for the belair spear I mounted out of desperation before I found the spear.
For whatever it is worth:
My fender mirror location for twin-strut mirrors is as follows:
Hooking tape measure on point of front fender closest to door hinge, I measure 8 1/2" to center of the rear-most screw of mirror base.
From the edge of fender where it meets the hood, I measure 3" to center of the rear-most screw, and 3 1/2" to center of forward screw.