Look here, this may help http://www.oldgmctrucks.com/photos/Sunvisor_Instructions3.gif and here, http://www.oldgmctrucks.com/page0024.html left column, third up from bottom shows instructions for chevy and GMC when you open that link, again near the bottom of the page, gives some details for the kind of visor you have and the fulton with the dog-bone brackets.

1948 GMC 1 Ton
In the Stovebolt Gallery
More pix on Photobucket