Marty, I dug into this subject a few years ago and after a large amount of help from guys on this and other internet forums and conversations with anyone who would talk to me about this odd happening, it seems that not all "J" code 68 Mustangs have a nine inch. Only those that were "GT" cars were routinely equipped with the 9 inch, and the ratio was typically 3:25 - 1. The single exhaust "J" code cars had an eight inch, normally with a 3:00 - 1 ratio. As an aside, the housing, differential assembly (pumpkin to many) the driveshaft and brake plumbing are the only different parts. The axles, brake backing plates and brake shoes and cylinders and hardware and drums interchange between the 8 and 9 inch assembly. It seems that the most convenient way to determine the axle size on a complete and driving car is to determine the axle ratio stamped on the door data plate. 3;25 gears normally are found with a 9 inch. And yes, there are exceptions to this but MOST of the time just a quick check of the ratio will give you a strong indication of the differential. Hope this helps you a little in your quest.
