Drilled step supports. The step actually goes through the lightening hole in the rib.  the support ends up being very close to centered on the lightening hole.

 

Alighted and drilled step attach bolt.

To make sure ribs are straight you attach the forward part of the baggage floor, and then cleco it to the seat back support so that you can get underneath it.

 

Painted and inserted steps.  Repainted after insertion.  Bolted in, and then attached padding to protect shins during build.

I did not drill through the bottom skin or baggage skin as I am planning on putting access plates in the baggage floor.