Just did some yesterday, probably the most important things are, 1. The bolts are centered on the gunwale, 2. The ends of the seats fit properly under the gunwale so the holes are not too close to the ends of the seat frame and, 3. Drill as vertical as possible to keep the bolts square to the seat frames. For spacers to level the seats, 3/8 brass nipples from your local plumbing store, hacksawed to the correct length work fine. You can clamp the seats in place but it's not really necessary as long you can hold it in place for the first hole, install a bolt in each hole as you drill them. A scrap piece of wood for a temp. spacer works.
