I made the decision to rivet the instrument sub-panel and front deck ribs in place today. The Vans instructions are not at all clear if you're meant to do this now, or later, however on reading ahead I cannot find any reason why it cannot be done here. My concern is I now have hindered access to the back of the firewall. I can still get to it, but just not quite as easily as before.
Some of the rivets are very difficult to get to. I'm convinced the Vans Engineers designed in a few "challenges", just for fun.
Instrument sub-panel and front deck ribs assembled