Tips on Ford Rears, by Dan Jones

Rear Widths:

65-66 Mustang 57 1/4 inches
67-70 Mustang 59 1/4 inches
71-73 Mustang 61 1/4 inches
77-81 Versailles 58 1/2 inches

Dave W. personally measured:

79 Mazda RX-7 57.5 inches
74 Maverick 8" 56.5 inches
75 Mustang II 8" 57.0 inches

From Street Rodder:

67-73 Mustang, Torino, Ranchero, Fairlane 9" 59-1/4 to 61-1/4
57-59 Ranchero and station wagon rears, 57-1/4
66-77 Bronco 9", 58"
77-81 Granada/Versailles, 58"
67-71 Comet, Cougar, Mustang, Fairlane, 59-1/4"
71-73 Mustang, 61-1/4"

From Fatman Fabrications:

70-77 Maverick 5 lug 8" 57 inches
57-59 Ford 9" 57 inches
75-80 Granada & Lincoln Versailles 58 inches
67-70 Mustang/Cougar/Fairlane 58 inches
71-73 Mustang/Cougar 60 inches
73-76 Torino 62 inches

Misc Sources:

64 Falcon 58" 
67 Cougar 60" 
67 Fairlane 63 1/2" (coil springs)
72 Ford Van 3/4 ton 68 1/2" 
73-86 65 1/4" 
57-59 Ranchero and station wagon 57 1/4" (narrowest 9" housing)
66-77 Bronco 58" but has 5-on-5 1/2 inch diameter bolt circle (can be redrilled)
67-73 Torinos, Rancheros, Fairlanes 59 1/4 or 61 1/4" 
67-71 Comets, Cougars, Fairlanes 59 1/4 inches

9" sources 

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

Housing Types

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 1/4" diameter axle tubes with flat tops

69-77 Galaxies (coils), Lincolns (coils), and late pickups (leaf) -
3 1/4" dimeter all the way to the backing plate, coil housings have
upper control arm mount

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
?? one year only 1/2 ton truck housing welded together from two halves L/R
63-77 Lincoln, LTD, Thunderbirds had 9 3/8" 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 gears 

Axles

28 spline axles cannot be shortened and resplined (they're tapered)
72 and earlier 31 spline axles are shortenable 
73 and later 9" (cars) have a 5-on-5 bolt circle and non-shortenable axles
67-73 Mustang axles identified by wheel flange:
oval hole - 28 splines
two large holes and counter sunk center - 31 splines