Transfer drilling through the firewall into the forward flange of the rib, it was obvious that these holes are too close to the web. There's going to be no way to dimple these holes.

...so, I fabricated a couple little plates that can be riveted to the firewall.

...thusly.

Also made up some blanks for bending some rivet-on flanges that will go the opposite direction as the rest of the rib flanges. These will rivet to the sub plates.

Bent them.

Cut off the existing flange.

...and drilled the new flange to the rib. Unfortunately, when I positioned this flange, I forgot to account for the thickness of the little plate on the firewall. If I were to reposition the flange, the holes wouldn't line up. Arg. Going to start over on this one.