Mark center line (purple) and cleco the end holes of the bottom longeron. Mark (red) all the other holes, remove clecos and using small square mark (red) layout line on the center line, center punch on center line. Pilot drill both longerons.
Clecoed to the skin every other hole, it was a little tight but the all went in, pilot drilled to straighten up the holes. Moved all clecoes over one hole and pilot drilled again. This process seems to work good for me. The fit is by the numbers.