When the factory did it, they used a hinge with a ~5/8 relief hole in it for the head of the bolt. They only needed about 9 attaching bolts, and they are 1/4-20 grade 5 with 7/16 low heads. The AMK master bolt kit includes them in their 66-68 kits.
PLEASE stop with the nutserts. Am I the only person that has ever had to drill out a nutsert? Hasn't anyone had a door mirror that they couldn't fix because the nutsert spun in the sheet metal? I can tolerate the hex style nutserts like the ones in the tailgate handle...but the round ones are horrible. Like most things today...they last just long enough to be a problem for the next guy. Once they spin, then you can't drill them out. Because they spin.
And STOP with the stainless button heads. Stainless threads love to gall. And the tiny little hex is not nearly adequate to turn a bolt. So they round out. And Stainless is really hard to drill.
If you are dead set on using a nutsert, please use fasteners that can handle adverse environments. Marine grade is a good idea. 20 years from now...it might be you that has to fix it. A brass screw in a steel nutsert would be my choice.
What's wrong with a bolt and a nut?