Got the inner seat rails *mostly* installed. Still need a few of the steel rivets to put them in place. The instructions weren't so great and didn't specify what rivets to use for attaching the non-stick plastic pieces on the rails, but countersink makes sense (otherwise, the seat runs on the rivets, which is not good!) and 3.2mm seems right, since that's the size of the holes! Also needed to countersink the holes that go in the bracket that the seat stop pins go into (the mechanism that keeps the seat from sliding back and forth).
Note added on April 29, 2023 - shouldn't have put those plastic parts on yet. They get riveted through the rails, some inside skins, and some support structures. I'll get to drill those out later...