You can find most of the specs here: http://fteufert1.home.att.net/pontiac/page2.htm
Just look under "rear axles". It's a comprehensive list from early 60's to about 79'.

But you'll have to double check your #'s as it should be a double letter Like "ZP". Sorry I can't say for sure, but I thought it was stamped on the housing where the axle tube comes in... not quite sure on that?

Once you have a letter code you should be set. Do you know if it's posi? Do you have any documentation (build sheet etc...)? You might want to get the PHS History. http://www.phs-online.com/
If you didn't know, for $35 you can get them to send you a history packet on your car.

Let us know what you find? Good luck!

Brett - 67 Drop Top 455!