[1960HDGMC]

Hey Jake, I ran into the hole with the slight notch in my truck. Only not this hole. The dealer punched a hole top center of my dash, and mounted a Buick clock. The whole install had me thinking it was a rare factory option. I think the dealer used a similar punch to get this hole up where a flat mounted generic antenna would ride pretty close to plumb. You did an excellent job of working the unit to the hole you have, but I sincerely believe that hole was not there when it left Pontiac.
The antenna I received looks to be a twin to the one in your pictures. The bases are the same to my eye. I used the template and it dropped in plumb, and bolted easily to the included bracket. Just now I went out and laid tape and measured the installed antenna on my truck. These pictures show the measurements I recorded. 7 1/2 " from front leading edge of cowl to center of hole, 6 1/4" from body crease line to center of hole. The hole on your truck appears to be way off from these measurements. Knowing LMC, they may have given you a template from a 1964-66 GMC antenna. If I could , I would opt for a flat based antenna and be done with it. But the only way this 90 dollar kit is ever going to fit as designed, is to tap a 5/8" hole at these coordinates, and plug the notched hole. No grinding/painting, and it will ride where it was originally supposed to. It will look as if it has always been on the truck. You would have a hole to fill and paint. You might hunt another cowl piece that has no hole and save filling the hole. Might be easier to get a good result. Good luck whatever you decide to do,GM