What a great accomplishment! It feels great to get the project up on wheels and the engine on. My 9A build log has lots of head scratching hours in it. One of them was something I think I see in your photo. After installing the engine mount, hanging the engine, running CHT and EGT wires and other assorted wiring and plumbing, I looked at that row of clecoes holding the bottom skin to the lower firewall flange and wondered how a bucking bar was going to fit between the lower engine mount tube and the flange. The short answer was that it wouldn't. As I think back on it, I can't think of a reason not to rivet the lower firewall flange before installing the engine mount. For future builders, this is one of the places where the plans should have a note: "Before installing engine mount, figure out how you are going to set the bottom row of rivets".
Keep building! One day, we will be reading about your first flight!
Flying since 7/26/13