BruceW here is a good discussion with explaination to figure the ratio out.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=003825#000000
The 12 bolt rearend for trucks and cars has been inservice for lots of years. If you look on the top side or front of the passenger side tube there might be a GM number? I guess the other question could be, is it really a 12 bolt rearend?
Good Luck


John....38 Chevrolet Panel Delivery
http://community.webshots.com/user/tobytailford