The drawing shows the doubler being riveted to the angle stiffner and on the forward side of the firewall. I've seen a few construction logs that put this doubler on the inside or aft side of the firewall. And riveted to the firewall but not to the angle stiffner. I don't know why they did this. I think riveting it to the stiffner helps make it secure.
The drawing shows that you drill out ONE (1) rivet to locate this doubler and drill a new whole through the angle. I decided to drill out two rivets and reuse them to hold the double. That meant I had to increase the height of the doubler by 1/2 an inch to 2". With a 2in height it spans the gap between two existing rivets on the angle stiffner.
So I clamped it and drilled it.
Then I match drilled the large 9/16 hole for the AN fitting. I used a step drill to do that.
