With the original ribs installed and the spacer blocks and new ribs clamped in place, I
transferred the holes through the firewall into the forward flange of the new ribs.
After some additional drilling, the new ribs were also drilled to the sub-panel.
I also trimmed the length of the F-643-1 center support rib per the plans. This rib
supports the top forward skin in the center. I'm considering making a large-ish cutout
in the top center skin which would span this rib to allow for a removable access panel.
Lots of builders have made a similar modification, but with two separate panels on either
side of this rib. It seems easier to me to just make one large panel.