I assume you have a dacron string and that you'll be shooting normal weight points, say 125 grains. If that's the case, 65-70# would work nicely. However, over spining another 5# will work as well. Your problem will be to find shafts long enough. Rose City, the only major manufacturer of cedar shafts today, mills them to 32 inches. Taking an inch of that length for the point taper leaves you 31 inches of arrow, not enough for a 31 1/2-inch draw. You need a shaft length of at least 33 1/2 if you don't change your draw length as Bjorn recommends. That will give you one inch of shaaft overhang behind the head for bow hand clearance. I also concur with his recommendation for Surewood shafts. I believe they will make them longer than 32 inches. Or, you can foot 32-inch cedar shafts to make them longer.