Installed upper and aft gussets to the firewall and floor stiffeners, respectively. It's a tight fit to drill the upper gusset to the upper crosstie at the firewall with it angled and all that. So, I marked the spot with the drill (drilled just a hair), and will drill it to fit with the firewall removed.
So... in order to remove the firewall, I decided to up-drill it first. I also mounted the upper firewall, and used the hole duplicator to drill that one center hole from the lower firewall to the upper.
Up-drilled all firewall holes to #30. Have not yet removed the firewall (ran out of time). Will then deburr and prepare for riveting.
Based on feedback from the Sonextalk Yahoo group, I am going to leave the aft fuse lower skin completely off. This will necessitate riveting the lower forward fuse skin EXCEPT for the row of rivets at the fuselage halves mating line.