I am new to the forum, and have been browsing around looking for Pontiac rear end info. I have a 68 400 Firebird and am trying to determine what I might need to rebuild, upgrade or simply replace the rear end. I have not disassembled it yet since I am planning on driving it a while before I jump into it. The only marking I can find on the axle tube is ZM. From what I find this is an indication of a 3.36, 8.2, 10 bolt posi. It also indicates it is a 4 pinion. I am not finding a whole lot of info on what that means. However, I did find one thread where they say it means there are six spider gears instead of four. This doesn't make sense to me at all. But I am not a rear end expert either. I see it was mentioned in a thread a few years ago about the 4 pinion, so I thought I would reach out and see if anyone could shed some light on the subject for me. Thanks!