Skip,
In agreement with DES, I would look at some guns like Fox, Parker, etc., and take some ques from them. A Parker stock tends to be quite thin in the wrist. But then, I've seen more Parkers with broken wrists than Nitro's. Nevertheless, imagine the "cheeks" or "panels" just aft of the reciever, then start narrowing from there. I'd use a rasp to start with, then on to some 36-40 grit, then finer and finer. Your wood is in good shape and should be fine at a thinner profile, IMO.