Mirrors should be mounted on the door.  A factory accessory instalation manual will show the exact locations to drill holes.  In 1964, the side mirrors were all dealler installed accessories.  the install manuals told the dealers where to put them so they did not interfere with the wingwindows operation.  people often installed them on the fenders(going back the the 50's era).  i suppose many car buyers back them told the deallers where to put them.
The correct mounting is what the accesory manual shows,  you should be able to track one down.  I have all the templates for the 64 chevelle, but not the nova bodys.