I made this mistake
I made this mistake early on, from not studying the issue carefully enough. Here's my first stiffener, riveted on the inside of the firewall in the cabin. Note that it bumps up against the angle stiffener, but there's no way to rivet it TO the stiffener:
When I got to the firewall forward part of the project and started working on it, it bothered me once I discovered that it should have been on the forward side of the firewall, so that it can be riveted to the stiffener angle on the firewall, just like Bruce has said. I also noticed that without this stiffener, the fitting required two thick stainless washers to provide enough thickness for the nut. Clearly, it was made with the intention of having a stiffener on the forward side of the firewall. So I made another stiffener out of steel for the forward side of the firewall. I had lowered my fuel exit point, too, to allow room for the starter solenoid, so it had an odd shape. Here's the finished installation:
I wiggled the fitting firmly with my fingers when I had only the aft (inside) stiffener installed. It was pretty firm. But all I can tell you is, with the additional stiffener on the firewall forward side, it is MUCH stiffer and I feel much better about it now. You may also notice that I replaced the standard blue AN fitting with one made of steel. The forward stiffener is also made of steel. I don't want anything made of aluminum forward of the firewall, if I can help it, especially if it has anything to do with fuel. There's another thread on here discussing the issue of fire in the engine compartment. Dan Horton has done some tests that are definitely worth reading about.
I have more pictures and more details about my installation on my web site, here:
Fuel Line Stiffener, Firewall Forward