I bought sunvisor brackets and arms, and the sunvisors for my 53. It had them missing when I bought the truck.
they are in now and look great.
I noticed that there is a hole drilled into the cab 6inches on either side of the rearview mirror.

What is supposed to go there?

Is it some kind of bracket the connect the sunvisor arms to?

 

nope, it may have been where the speaker for the radio was positioned if your truck had an original radio,
abe
(show us some pics)

 

Mine has a couple of 1/2 hinges pop rivetted there that I have no idea about. Who know what someone might have drilled holes for, but unlike sunvisors of today the OEMs didn't have inside clips they just fluttered with the bumps. Think of them as butterfly wings... LOL!!!