I inserted an entire row of rivets and taped them in place before installing and riveting the stiffeners in place. The first picture shows the rivets before putting on the tape. The second picture shows the rivets after they have been bucked and the tape removed.

After the stiffeners I installed the gas cap flange. I was very liberal with the sealant on the back side of the pieces. Adds weight but gave me piece of mind that I have blocked all possible fuel escape routes.