I have a 67' Mustang that came with a 289 and a 4 speed. I would like to know what type of rear end I may have in it as I do not know the tale tell signs between a 8" and a 9". I really need to put different gears in it. Because the previous owner obviously had it re-geared. I am currently running about 2,800 rpm at 60 mph. And passing anything but a gas station. I would like to upgrade to a 9" if I am currently running an 8" as I am now running a 351 mildly built, and do not want to overwhelm the axle with the wide tires I run in back. I have access to a 78' cougar xr7 that has a 351 And I would believe a 9" as I could have it reworked to fit the Stang. Although I can have it for free it is 400 miles away at my dads. And I can not eyeball it side by side with my Mustang to see if there is any real differences. Or if I am running a 8", will it stand up to the 351 and 10" wide tires? If that is the case what ratio should I be considering?