I had a minor epiphany on the way to work this morning. It occurred to me that I could use the new skin as a guide for drilling out the rest of the rivets. I clecoed the new skin to the old one around the edges, and through some of the rivet holes I'd already drilled. Worked like a charm! In less than 2-1/2 hours I drilled out every rivet in the skin and final drilled every open hole. The new lower skin is ready for de-vinyling and deburring.