Riveted the two inboard floor stiffeners to the center engine mount angles on the firewall, and to the cover support ribs on the F-704 bulkhead. The very aft rivet joining the aft end of these two stiffeners to the cover support ribs will require use of an offset rivet set, and therefore a third hand to buck (see photo 2).
The rivet in the lower firewall transverse angle through the stiffener in photo 1 is just a locator rivet, and is not set.
The outboard stiffeners only rivet in assembly with the forward bottom skin, and will be installed later.