If we had to do it again, we'd wait on the stiffeners until after we installed the recess - and do the recess while we were building the firewall. We originally built the firewall to plans, with the stiffeners as shown on the drawings. Then a year later, when we added the recess, we had to cut the stiffeners in making the opening. I don't think the stiffener design is particularly critical (my opinion), and you could easily modify it to match the recess. I spent a lot of time staring at Randy's firewall pictures when I cut the hole for the recess!
Paul