I would do a complete frame up 16" oc and apply the hardieplank horizontally with a 6 1/2" reveal, level all the way around, starting a little above grade to prevent water problems. Then install a cap of 5/4 x 6 Miratek around the perimeter, mitered on all ends for pretty. The Miratek cap will help shed water over the sides and down the hardieplank.