Cleco'd on the floor pans to prepare for drilling the F-715 ribs to the skin. Started drilling and clecoing the holes, using the centerline drawn on the flange for reference. After the first couple of holes it became very difficult to keep the centerline visible in the holes. Removed forward and seat floors for better access to hold the rib in place for drilling. As I aligned the rib on the holes the top side bowed out, well beyond the edge of the floor. Tried fluting the top of the rib, but this did not fix the problem. The rib seemed to be too long for the space, even though I had previously removed some material from the front end before I drilled it to the F-704. Removed the cleco's and the rib fit better. Removed more material from the front of the rib and the fit improved even more. However, the holes for the F-704 were now mis-aligned. Ordered new F-715s from Vans.