When it came time to fit the fireall sub panel brackets to the outboard firewall stiffeners, there were a couple complications. The first is that these rivets prevent the bracket flange from laying flat on the stiffener flange. I took this picture to get a good measurement of the spacing of the rivets.

...then laid the spacing out on the appropriate stiffener.

...and drilled some clearance holes to go around the rivet heads

Later, I realized the second problem with fittng these stiffeners. The turned-up edges of the firewall weldment gussets was going to prevent the pop rivet gun from being able to fit the stem of a pop rivet, so I decided to shorten the outboard vertical brackets about 1 3/4 inches on both the top and bottom.

The diagonal brackets went pretty well. Several hours later, all brackets are fitted to to the firewall.