Once the front two formers are installed you can install the landing gear plate. There are notches in the fuselage sides for fitting this plate. You were careful about lining up the balsa fuselage sides with the ply doublers weren't you? Note the etched line on the gear plate. It goes on the inside and should align fore and aft. This will be important shortly. It shows up later but there is also a ply doubler that reinforces the front wing mount.
Now you can glue the firewall to the forward fuselage floor. The laser cut floor piece is used on both the nitro and electric version. The nitro version firewall is shown but the kit includes a laser cut firewall for mounting a brushless motor. The firewall and the floor are notched to fit together. Use a square to set the firewall angle at 0 degrees to the floor.