I see, by the Chevy Truck picture you posted, that you have studied the issue. So have I.
I am looking for one also for a 1960.
The location of the hole changed during 60-66. These changes are best studied with available Chevy info. Unknown if GMC followed the same changes. The changes may have been made due to signal reception, hood clearance, extension of antenna under the sheet metal or for better bracketry. At any rate, I see two different new right side extensions being sold on Ebay. Both have what I will call a "top" hole. These top holes are in a little different location but still not an the "side". One of them says it's specifically for GMC. I do not see any new extensions offered with a side hole shown in your Chevy pic. The picture you posted of the white Chevy has a supposed N.O.S. 1960 antenna. It is angled for a side hole and has a template that may or may not be deciphered and used (for a Chevy). Now that I see all this, I'm going to find an antenna that works with the hole I have and call it good. This is a lot of BS over a small issue I know. But you and I are stuck with just getting it done.
I will say if I bought an antenna from Classic Trucks for a 1960 and it did not sit straight using their fender extension, I would get money back. If you say it won't fit your truck, they may say your hole is wrong. Maybe they come with a template. I would expect that we need the type that has a little angle adjustment to it. Good luck to both of us. I have searched the internet for GMC and most pictures show the "top" hole on the flat part of the extension, like yours, like mine. Go figure.
Sorry to say but the cheapie universal are the only ones I see so far that mount on the "flat".
You could possibly cut the bezel, the rubber may cover your cut. It may chip the chrome. You can find a flatter bezel from some other brand truck or car. I have spotted a few car antennas on Ebay that look promising. Maybe someone here will straighten us out on this.
http://67-72chevytrucks.com/vboard/s...d.php?t=553274