After changing the eyeball backing plates to the aft side of the recess (how did I fail to notice that yesterday?), I clecoed the recess to the firewall and proceded to rivet the recess and weldments to the firewall.
I then located the hole in the right tunnel rib for the scat tube support and lhen match drilled the support to the rib. After dimpling the screw holes in the rib and support, I also dimpled the rivet holes in the support and two nutplates. After deburring the edges of the support, I riveted the nutplates to the support.