Hello, I have restored a 51 and a 54 Chevy 5 Window. On the two I did, the antenna was mounted on the right curved side of the cowl. If you look under the curved part of the cowl from underneath you should see a metal plate that gives this antenna some support. If I remember right, the metal plate is right in the curved part slightly below the top of the cowl. Take a look, hope this helps.

PICTURE OF 2 TON TRUCK MOUNTING HOLES FOR MIRROR .PICKUPS USED 3 MOUNT BOLTS