Reached under seat base with marker to outline edges of SSO supports on the bottom of the seat base. Disassembled the seat base and marked the inner bolt hole locations using plans for distance from the center channel. The plans measurement did not match the physical hardware for distance from front to back, so I used the actual measurement of the hardware for that dimension. Pilot drilled the bolt holes. Laid out and pilot drilled the A5 rivet lines for the SSO supports on the seat base. Reinstalled the seat base and drilled all holes to final size into the SSO supports. Test fit the hardware and it all fit ok. Disassembled, deburred, and treated the parts. Reassembled and riveted the innards of the SSO support L angles. It is actually possible to get the hand riveter under the seat base through the top holes in the seat base and see what you are doing by looking through the lightening holes in the front. Don't know if I could of gotten it done if my arms had been any thicker though. Then I riveted the seat base along the back and on the SSO support lines. Didn't rivet the front support as I think I will need a little play to get the pilot side in place. Finally, I bolted the SSO hardware, including bottom cushions in place and added the connecting wire. Tested to see that the actuator releases both sides. So far, so good.
Note from future self: While working on the seat supports, it would have been beneficial to drill and rubber grommet some holes on each side for future routing of wiring/pitot-static lines etc. Having not done this at the time of seat building, I will need to provide some sort of protection for these items routed through the lightening holes.