The brief plan description for this work did not seen like the right approach to me. It seemed more logical to me to establish the interfaces to each end of the inner stiffners to the firewall and center section and then drill the skin to stiffeners. That is the process I followed. Ed Lynn has a nice description of this approach in his build log.
The first thing I did was to mark the for/aft location on each end of the inner stiffeners to protect end margin for each of the end holes. The stiffeners come shorter then the distance between the firewall and center section. I then clamped them into place and drilled the forward and aft holes.
One thing I missed when building the firewall was the three holes needed for connecting the inner stiffeners. I found that drawing and built a drill template for locating those holes in the installed firewall stiffeners.
