Thats the standard C7AW case...used all the way thru the production from '67 to the 80's. ID by the single vertical reinforcing rib the the top of the case. The differential IS a posi..an EquaTork. You can tell by the recessed bolts holding the ring gear on. A TracLoc would look the same, except the face of the carrier would be machined, not 'as cast' like the one in the photo. If only the pinion seized, more than likely I would say somebody got a bit too happy with the impact tightening down the crush sleeve and collapsed the bearing. IF it seized, the pinion would probably be heat stressed..get rid of it and check the case for signs of severe heating (bluing) around the bearing pockets!
Setting up a new R&P is not hard...just time consuming. Any of the old shop manuals will explain it ('65-6 Mustang manuals abound, and are cheap). If you're running a TKO with the .63 OD, I would recommend a gear set around 3.73-3.89 which would give you a final drive of 2.35/2.45 which is great for cruising. Figuring a 295/50/15, or similar 26.6" tire, you get +/- 2166RPM at 70 MPH (3.89 gearset).
