Tonight I deburred the edges of most of the gussets and spacers so they'd fit when clecoed in place, fabricated the F-601E stiffener for the bottom edge of the recess opening, aligned and clamped the F-601J angles and their associated spacers and gussets, shortened and radiused the the top ends of the diagonal and center vertical stiffeners for proper fit, and clecoed all those parts in place for match drilling.
After a short search I found pre-cut stock for the spacers for the upper ends of the diagonal stiffeners (Bag 1982-1). I will need to locate and drill one hole in each of these parts, radius one edge, and clamp them in place before match drilling. I still haven't located the F-601TD brake line pass-through doubler that gets installed on the lower left quadrant of the firewall.
Gussets and spacers are clecoed for alignment, then taped in place.
F-601J angles clamped in place, ready to match drill.
Still need prepare the spacers for the upper diagonals, and find the brake passthrough plate.