Check the directions (page 12) - the rabbet is only needed on the last scale foot of the plank ends.  Sanding will work as shown in the photo from the instruction book (below).   However, the gains on this boat are different. Each is a sloping rabbet, rebate or recess, cut along the last scale foot or so at each end of the plank. This gain is a scale inch wide. You can measure this using the scale rule (page 7). Mark this distance on the edge of a paper strip, then transfer this measur