I've never seen a "factory" (which is usually really dealer) antenna installed anywher except the stock location. That "original" antenna goes hard up on the beltline and actually is just a whisker into the beltline for proper fit. Here's how the factory manual shows the setup with a visor intalled on a '54 GMC:
[img]http://www.oldgmctrucks.com/photos/stovebolt/54sunvisor.jpg[/img]

Here's a different look:
[img]http://www.oldgmctrucks.com/photos/stovebolt/RadioAntenna.jpg[/img]

http://www.oldgmctrucks.com