All the parts started to get permanently attached tonight.
First came the cross brace that supports the tops of the rear seats, and then came the rear longerons (and the misc parts for the baggage door seal that gets attached to them).
Next came the front ribs. These were a bit challenging, as there are a whole lot of rivets that join the side channels to the steel firewall brackets, and some of them are in awkward locations. However, with an offset rivet set and my trusty tungsten bucking bar, everything was set nicely (and I forgot to get a picture of it!)