Pulled out all the firewall parts and deburred them. Cleco'd together just to get a feel for the assembly and confirm everything is all final sized. Then disassembled and countersunk all the holes that contact the firewall. It was a lot of countersinking. Not much to show except a pile of parts. Did rivet the inset box together. It that's not much. Did notice than Van's sent me two right WD-1002-PCs. The left one is mislabeled. It's actually a right. 3/19: Read ahead and saw that tie of the nut plates are mounted on the front side of the firewall. Was initially thinking that is contrary to the previous direction on 27-2 to dimple everything for flush rivet in the front of the firewall. At this point I have already countersunk the stiffener to accommodate a dimple in what is maybe the wrong side. The. Realized if I can just dimple the nut plate, but backward from what I normally do, the manufactured head can sit it the nut plate dimple. Had to modify the male side of the dimple to get in there but think it will work. Then it warmed up enough this afternoon I was able to prime the firewall stiffener parts. Once cured, I cleco'd everything together. Realized that it was going to be very difficult to rivet the rudder pedal support bracket after the firewall recess is already riveted in place. Proceeded to river the bracket in place ahead of time. Crossing my fingers that is not a mistake. [spoiler alert - it was a mistake.]