A while back there was a guy on the Racing and high performance forum here that put a set of passenger 12 bolt gears in his truck 12 bolt. He said that he found the pinion bearing to make it work. I don't remember his user name, but you might try a search or a post over there.
Here I just found the thread.....
http://www.67-72chevytrucks.com/vboa...ight=car+gears