Installed stiffeners on firewall and side skins. It took longer than expected, because it was difficult accessing the side skin stiffeners from inside the aircraft. I didn't follow the instructions regarding trimming the side stiffeners inside the aircraft - too difficult! I drilled the side stiffeners off completely and trimmed them on the bench. Also managed to do the job without removing the gascolator bowl - just the drain. I used a 6" drill to get to the lower 2 holes behind the gascolator. I used the close quarter rivet puller in those locations and a couple of other tight spots. Otherwise, the pnuematic rivet puller did the job. Overall, not a difficult job, can be done by one person.