The firewall rivetting gave me little more headaches then expected. Because of the thickness of the primer, I needed a rivet length slightly longer than the AD4-5 rivets called for in the plans. I ended up with a 5.5, which is a size that does not exist. The 6' were too long and bent over while setting. As these are quite important parts, I wanted the rivet size to be perfect. The 5' would end up being either too thin, or too narrow. 
In the end, I got a rivet cutter from Hugo that I used to cut 6 rivets to 5.5. This worked quite well. You just have to pay attention that the rivet head remains horizontal. The rivet cutter has the tendency to leave the head at an angle after being cut.

I had some problems deciding on which technique to proceed with on the firewall. I did some attempts with backrivetting but this worked pretty bad on the sides where the side supports meet the firewall. You have to backrivet for the center rivets but you can also squeeze the ones on the sides using the G-yoke and sliding it in from the side.

Here is an example of my setup. Put a clamp on the side, put the rivet in, slide the squeezer in, and then put another clamp on the left side of it (not on picture as I only have 2 hands :) )

Once the technique got refined, things went pretty smooth.

 The following shot shows some backrivetted AD3 rivets on the F-601-D-R stiffener. Important here is to press the stiffener well against the firewall.  When the rivet gun is perpendicular to the work piece, the rivets end up very nicely. 

From the back, you can see that I put some backrivet tape over each rivet. Pay special attention to the order in which you do things especially towards those corners.

Another backrivet example on the F-601TD brake plate. Tape the rivets with rivetting tape.

 Use the backrivet set to shoot them at a pressure of +-37 PSI.

Et voila... perfect

 And very nice on the other side.

Firewall rivetting work will continue in the next session.