If you have not drilled holes yet: I suggest you look for the mounting support brace under the cab/cowl.

When I took my west coast mirrors off to install the long single arm mirrors, I found the braces under the metal had been filed in with "bondo." i popped out the bondo and mounted the single long arms with the braces as it came from the factory. No new holes were needed! I have a 1951 so yours might be a little different. But it s worth looking before drilling...

BTW: I have those long mirror arms inplace for over four years and they have not come loose. Now I did slip once while waxing the top of the cab and nearly broke my neck falling and`catching myself on the drivers mirror arm. I had to readjust that one as it moved.