Finish riveting the bottom skin. Still not sure about the 915 outer seat ribs. I have not drilled the ribs nor dimpled the holes. After much debate and inspection of the plans I decided to match drill the bottom skin to the outer ribs. There was no furtther mention of the ribs in the instructions so I felt it was time to do it.
Mounted the seat bottoms and match drilled them to the ribs. Everything seemed to fit properly even the 915 ribs had just a little clearance (.032) and it all looked good.
