Larry wrote:

---

if you have 57" backing plate to backing plate, the total width from wheel to wheel will
be @ 63"

---

 Not always true! a car equipped with 11"x 3" brakes drums can be the same axle to axle measurement as a car equipped with 11"x2.5" drums. The difference will effect the backing plate to backing plate measurement.  (brake shoe widths come in 2.25, 2.5, and 3" widths) 

http://www.fordmuscle.com/archives/2002/02/nineinch/

9 Inch Ford Rear Axle Information

Many people swap Ford 9 inch rear ends into their hot rods, muscle cars and drag cars because they're readily available, are somewhat strong from the factory and can be strengthened easily. The center section of the 9 inch rear end is removable allowing you to swap gears or differentials on a bench. Nine inch Ford rear ends came in many widths and configurations. Depending on the application, the rear end may be heavier duty, be made of high performance materials or have a locking differential. Axles came in 28 or 31 spline versions from the factory. Typically the more splines the better up to a point. You'll commonly see 9" race axles with 33 or 35 splines. The differential spline count has to match the axle spline count. Gear ratios varied based on the engine, transmission and vehicle type. Center sections, or 3rd members, are completely removable and were generally made of cast iron. High performance 3rd members were made of nodular iron and hold together better than standard cases. These types are hard to find and often command more money. You can usually nearly buy a new high performance case for what you have in a good original nodular iron case.

9 Inch Ford rear differential

Most 9" rear differentials will be the open type. To recognize a Locking 9" differential, look for a metal tag on one of the bolts that holds the center section to the housing. There is a gear ratio stamped on the tag. For example: The tag will say: 350 for 3.50:1 ring and pinion gear ratio. If there is an "L" in between, you should have a locking or limited slip differential. A gear ratio with a locking or limited slip differential would look like this on the metal tag: 3L50. Sometimes the period is on the tag as in 3.50, depending on the year and model.

There's nothing to guarantee that it is the stock differential if you're buying used. It's best to verify for sure.

Common 9" Ford axle widths

65-66 Mustang 57.25 inches
67-70 Mustang 59.25 inches
71-73 Mustang 61.25 inches
77-81 Versailles 58.50 inches
74 Maverick 8" 56.50 inches
75 Mustang II 8" 57.00 inches
67-73 Mustang, Torino, Ranchero, Fairlane 9" 59.25 inches to 61.25 inches
57-59 Ranchero and Wagon 57.25 inches (narrowest 9" housing)
66-77 Bronco 9", 58 inches (has 5-on-5 1/2 inch diameter bolt circle)
77-81 Granada/Versailles, 58 inches
67-71 Comet, Cougar, Mustang, Fairlane, 59.25 inches
64 Falcon 58 inches
67 Cougar 60 inches
67 Fairlane 63.50 inches (coil springs)
72 Ford Van 3/4 ton 68 inches
67-73 Torino, Ranchero, Fairlane 59.25 inches or 61.25 inches
67-71 Comets, Cougars, Fairlanes 59.25 inches

Where to find 9" Rear Axles

67-73 medium and big block Mustangs and Cougars-66-71 Fairlanes, Torinos, Montegos, Comets, and other Ford intermediates with big blocks
57-59 V8 Fords and Mercurys-77-81 Lincoln Versailles & Trucks

Types of 9" Ford axle housings

67-73 Mustang/Cougar - light duty, thinnest housing material, small axle bearings, 28 and 31 splines
57-68 passenger car and 1/2 ton truck - medium duty, stronger than Mustang type, 28 and 31 splines Ranchero/Torino - heavy duty thick wall housing, 3.25 inch diameter axle tubes with flat tops
69-77 Galaxies (coils), Lincolns (coils), and late pickups (leaf)- 3.25 inch diameter all the way to the backing plate, coil housings have upper control arm mount.

How to recognize 9" Ford housing centers

57 - no dimples, flat center band up the center of the rear cover, bottom drain plug
58-59 - two dimples on back of housing, flat center band, some had drain holes
60-67 - two dimples, flat center band, oil level hole in back cover

63-77 Lincoln, LTD, Thunderbirds had 9.375 inch centers, housings were cut away at the gasket surface for ring gear clearance, one curved rib at the front top portion of differential. Strong, but no gear options.

9" Ford Axle Facts

28 spline axles cannot be shortened because they're tapered (except 69-73 Mustang and Cougars).
72 and earlier 31 spline axles have the ability to be shortened
73 and later 9" (big cars) have a 5-on-5 bolt circle and the axles cannot be shortened