Hickory is not really suited to a D shaped profile, as it is strong in tension but weak in compression. Thus, it's heavy use as a backing material. Plus, it's pretty touchy in regards to humidity and moisture. Actually, it's a downright sponge. If I were building a 66" bow at that heavy of a weight, I would certainly make it a flatbow.
Lookin' forward to seeing what you come up with!