Greetings Bruce,
Yes, there are guidelines for thinning black walnut stands. However, I would strongly encourage you to contact your local DNR forester or professional consulting forester to assist you in this endeavor.
Unless you are astute in the ways of identifying desirable crop trees, your best bet is to receive on-site consultation regarding the thinning your black walnut trees.
References:
http://www.ncrs.fs.fed.us/pubs/misc/walnut/p98_101.pdf
http://www.ncrs.fs.fed.us/pubs/wn/wn_3_03.pdf
http://www.ncrs.fs.fed.us/pubs/wn/wn_3_04.pdf
Hope this steers you in the right direction!