Last edited Jan. 29, 2004 The driveshaft can come from many (not all) 1967 to 1970 Mustangs with a V-8 and manual transmission. Those from the all 6-cylinder engines and the 289's with the 7 1/4" differential won't fit either (the yoke span is too small and in many cases the shaft diameter is too small). The table below shows which ones can be used. The shafts from any automatic is much too long. Those from other cars, including the Cougar, are too long as well. Driveshaft length is measured from the center of the eye in each end of the shaft. The yoke length is measured from the center of the eye to the end of the "splined tube".
There are a few choices for differentials. First, there is the 8" or 9" ring gear, with the 9" being stronger. Almost all non-high performance 289 and 302 engines came with 8". All 351's (C, W, M), 390s, 427s, 428s, 429s, 460s came with 9" gears. I believe that you could get a 9" from the factory on any V-8 if you ordered it special. Additionally, there are two choices for axles, those with 28 splines (lower performance) and those with 32 splines (high performance, including racing). Axles with 32 splines were also available for the 8" axles, and were used for high-performance applications. Lastly, you can get an "open" differential or "closed" (locking) gear set. The open will allow one wheel to spin while the other is sitting on the ground not even moving. The locking type keep both wheels turning at the same time and nearly the same speed (although they aren't going quite the same speed in a corner). Now, what do you put in your Mustang? The 8" housings are the easiest to find and least expensive. They are available in Mustangs, Falcons and Comets from 1967-1970 and Granadas from 1975-1979. The 9" housings are found on the 1967-1971 Mustangs and Cougars, again these are completely interchangeable (spring perches 43" center-to-center). I got my 9" housing from a 1971 Cougar that had been gutted for it's high performance axles and differential. I used the gears, axles and brakes from the 1971 Mercury Montego (spring perches 44 1/2" center-to-center). The 1971 Montego and Cougar differentials are about an inch wider than the 1967 differentials but the spacing of the spring perches is exactly the same as the 1967. The 1967-1970 Mustang, Falcon and Cougar, and the 1975-1979 Granada are all interchangeable in that the axle widths are the same (within a 1/2" or so), they all have leaf springs, and the spring perches are all in the same location. You can simply remove the 7.5" differential that came with your 6-cylinder and bolt the new one in (See the brake section for some brake line modifications). You can drop in a 1971 differential also, just remember that it is an inch wider at the brake drums. If you happen to find a complete differential that meets your needs, great. But they are hard to find without spending a fortune. I bought pieces and assembled my own. Here is how. The gears are contained in a "third member", or "pumpkin" that is easily removed as a complete unit (pull the axles first). You can get them from so many different Ford models and years I can't even determine which ones won't work. Especially look at the F-100/150/250/350 pickups and Econolines (these have some dynamite gear ratios routinely); full size Lincolns, Fords and Mercurys; Torinos and Rancheros, etc. through the 1970's. They are very interchangeable. However, you can't mix an 8" gear-set with a 9" housing or the other way around. So be careful and read the tags and learn how to recognize an 8" and 9" third member. Similarly, if the gears are set up for a 28 spline shaft, you can't install the 32 spline, high performance shafts. Figure out which you want ahead of time and get ones that will fit. The 28 spline shafts are quite strong and, unless racing, they will undoubtedly serve you well for a long time. The Econolines and pick-ups have the (numerically) higher gear ratios. The table below will help you find which ones to look at. Passenger cars are much more limited in terms of available ratio, especially if you are getting the 3rd-member from a car with an automatic transmission and are installing it into one with a manual transmission. Automatics are (numerically) much lower than the manuals. (See the tables below.) I got my gears out of a 1971 Mercury Montego, the same car from which I got the engine. It is a 2.75 ratio. Not especially high, but with the high first gear ratio of 3.29 and low overdrive ratio, it is pretty good. Axles are the easiest part of the interchange unless you are looking for the 32 spline shafts. The 28 spline axles are the easiest to find. There is a difference between left and right (lengths are different) and and some models of cars have different length shafts. I have a table with some limited interchange information below. The 32 spline axles are very difficult to find in a junk yard, you need to find a Cougar, Torino or Mustang, which are usually picked over before you arrive.
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