Since I had already installed the nutplates in the inboard wing ribs I needed to very accurately locate the holes in the bottom skin. To do this I fabricated a dimpled hole duplicator tool.
Looking through my scrap box I found a miss-drilled seat belt anchor. It's steel so the pilot hole won't wallow out as quickly. I trimmed a strip of 0.025 aluminum to more or less match the shape and drilled it to the seat belt anchor. I then drilled a #40 pilot hole through both pieces. I then enlarged and dimpled the hole in the aluminum for a #8 screw. The idea is to slip the dimpled aluminum strip between the bottom skin and the wing rib and let the dimple find the dimple in the rib, then drill through the pilot hole in the steel. It was more difficult to precisely locate than I anticipated, but seemed to work OK. I will know for sure after I dimple the bottom skin holes and reinstall the wing.