An engine that will swing a 16 x 8 with any authority should do ok with an 18 x 6 prop as the loading is approximately the same. You are just trading diameter for pitch. The 18 x 6 will have more static thrust and less airspeed. Just the thing sometimes for a big, slow airplane. The APC props don't look very "scale" sometimes, but they are very efficient and what I always use and highly recommend.

Good luck,
AV8TOR