There is a TECH TIP on this topic that I followed when I insatlled a mirror on the right side of my '54.
Following the TECH TIP, I removed the carboard glove box and found two olbong holes in a backer plate that was already part of the cab. I drilled holes through the oblong holes in the backer plate and mounted the mirror.
I don't know about the mirror on the drivers side, but I imagine the same holds true.
To find the TECH TIP go to the Tech Tip page on this site. Scroll down a bit the the FAQ Page. Click on FAQ and you will see a "How Do I...'' section.
"How do I Install a rear view mirron on my AD truck is the 8th tech tip down.
Good luck
