Attaching the firewall turned out to be time consuming. I had to adjust the length of the longerons many time, fit the longerons and re-do. I guess I was pretty conservative in my adjustment of the longerons length, so it took many iterations. After being happy with the fit, I started drilling the fire wall, that took some time and many drill bits, since this is drilling through steel.
We attached the second skin. Then we realized we need to detach both skin to form the bend for the intersection with bottom fuselage. Ah well, we removed both and worked on the bend. We did a bit of digging in this one and we realized that people had issues with cracks forming at the bend. So, we were very careful, sanded the edges very well before starting to bend. The bends went great.
Then we went ahead and drilled the longerons. All went great.