Last summer I installed an 8.5 from a 75 Ventura into my 65 GTO. A few weeks ago I purchased an 8.5, at the junkyard, from a 72 Nova. Both these are the shortest 8.5's at around 60 .5 inches(going from memory). The 70's firebirds and camaros had longer axles makeing them wider. Here is a web site with some info
http://www.oldsmobility.com/10bolt-tech.htm