I've read that it's best to back rivet the firewall since the stainless is pretty easy to ding and bend up. My big back rivet plate is a bit too big to be able to do all of the rivets, but it'll work for the vast majority. It sets in between the firewall flanges okay, but because of the curve at the top of the firewall, the plate can't quite extend out to the vertical row of rivets on each side.
Thinking I was being efficient, I put all of the rivets in and taped them with rivet tape so I could flip the firewall over onto the back rivet plate just once. This turned out to be rather inefficient, since I found a lot of the rivet lengths the plans called out were too short for my liking. Some were fine, but most of them going through more than two layers ended up being one size too short. I ended up replacing a bunch of them with longer sizes.
Flipped over onto the back rivet plate and ready for all of the bits.
The nice thing about back riveting is how fast it goes! It probably only took 10 minutes to finish everything except for the corner brackets. There were a number of rivets in those that I had to finish with the gun and bucking bar. The firewall recess will be riveted on as late in the build as possible, since it provides a convenient space to reach through when mounting all sorts of things inside the cockpit.
While doing some research, I just happened to stumble on the fact that the fuel pump doubler (at the bottom of the firewall in the middle) is not actually necessary in my case. The plans and instructions say to put it in and don't give any indication that it might not be needed. It turns out, that doubler is only used for fuel pumps in a carbureted setup. For an injected engine, the electric boost pump is bigger and is mounted on the floor of the cockpit in front of the seats instead. When I discovered this, I asked around, and it turns out it's a common oversight. Since the holes were already drilled and dimpled, I went ahead and attached the doubler. It's possible I'll use it for mounting something else down the road. If not, I'll just put bolts through the open nutplate holes and seal it up.
