Here's some info I found that I thought would be worth sharing.
Every once in a while someone is asking what diff out of what car and how long... 
Originally created by Chrysler in 1957, the 8 3/4-inch rear was used in many Chrysler cars and Dodge trucks through about 1974. These differentials have a removable carrier, which, like the Ford 9-incher, unbolts from the front. It's also called a "banjo-type" differential assembly, because that's basically what it looks like from the center housing out to either axle.
Ten bolts retain the carrier, and the pinion spline can be either 10-spline or 29-spline. Obviously, the ring gear is 8.75 inches in diameter. Identification of these rear ends is pretty straightforward. There are only six identifying part numbers relating to a true 8 3/4-inch rear. Some of these you will probably never see, because they have not been in any applications after 1964, but they do turn up occasionally. The casting numbers are stamped into the center housing, on the driver's side.
Casting numbers 2070742 (used from 1961-1969) and 1634985 (used from 1957-1964) identify the 1 3/4-inch-diameter 10-spline pinion shaft, and are commonly referred to as the "742" case.
Casting numbers 2070741 (1964-1972) and 1820657 (1957-1964) identify the smaller 1 3/8th-inch-diameter, 10-spline pinion shaft, and are often called the "741" case.
Casting numbers 2881488 and 2881489 were used from 1969 through 1974 and were the only cases used after 1973. This is the most sought-after 8 3/4-inch version, and is called the "489" case. The pinion shaft diameter on these rear ends was 1 7/8 inches, and these pinion shafts were either 10- or 29-spline.
The increase in diameter of the pinion shaft also relates to the rear-end assembly's strength. Normally, the 741 cases were used in A-body or lighter cars with smaller engines. The 742 cases were normally for mid-sized models or higher-horsepower engines. The later model 489 cases would be used in heavier cars or higher-performance engines.
Ring and pinion ratios are primarily identified by a tag on one of the carrier mounting nuts, and the ratio followed by an "S" would indicate the Chrysler "posi"-style Sure-Grip assembly.
There is, in most cases, another tag identifying the limited-slip additive and the proper type gear oil that needs to be used in this assembly. This tag would also be attached to one of the 10 carrier nuts. Many ratios were available as options from Chrysler, but the more popular passenger-car ratios were:
2.76 (47-17)
2.94 (47-16)
3.23 (42-13)
3.55 (39-11)
Truck ratios were available as tall as 5.57. One of the advantages of using the 8 3/4-incher is that there are no carrier breaks, so the ring and pinion you do find can be changed to any ratio between 2.76 and 5.57 without buying an additional carrier. Another advantage is the fact that the non-limited-slip rear assemblies can be modified to a Sure-Grip limited-slip differential just by exchanging the carriers. The only year the Sure-Grip wasn't offered in the 8 3/4-inch configuration was 1957. In the earlier applications, from 1958 to 1969, a Dana limited-slip unit called a Power-Lok was used. This assembly incorporated clutches to give the differential its locking capabilities. From 1969 to 1974, Chrysler vehicles used a spring-loaded cone assembly called a Borg-Warner Spin-Resistant assembly to provide the locking ability of the differential.
Finding one of these rear-end assemblies shouldn't be difficult. Things to look for when sourcing this unit are the casting number, ratio, whether it's already a Sure-Grip, and whether it was made before or after 1964. The pre-1964 axles were tapered and held the brake assemblies in place by the use of a key and locknut. Later applications used flanged axles, with the ability to make slight adjustments in width by use of an adjustable axle retainer used on the passenger-side axle. The later-model axles are also not tapered, so it's possible to have them cut and re-splined to fit narrower-width applications.
Here's a list of cars and trucks you can look at to find one of these differentials:
1964-1976 Dart, Demon, Scamp, Duster, Barracuda, Valiant
1957-1974 full-size Newport, New Yorker, Imperial, Polara, Monaco, Fury, and 880
1962-1974 Satellite, Coronet, Charger, R/T, GTX, Belvedere, Road Runner and Super Bee
1970-1974 'Cuda and Challenger
1958-1974 D100 and W100 pickups
1964-1970 A100 vans
1971-1974 B100 and 200 series vans
There were several different rear-end housing widths used on these models. All measurements are from brake drum to brake drum:
1966-1972 A-bodies - 57 1/8th inches
1962-1963 B-bodies - 58 1/2 inches
1964-1967 B-bodies - 59 1/2 inches
1970-1971 Imperial - 59 3/4 inches
1968-1970 B-bodies - 60 1/8 inches
1970-1974 E-bodies - 61 5/8 inches
1965-1969 C-bodies and 1965-1970 A100 vans 61 3/4 inches
1965-1966 Imperial - 62 inches
1967-1969 Imperial - 62 5/16 inches
1970-1971 C-bodies and 1971-1974 B-bodies - 63 inches
1965-1971 D100 pickups - 64 inches
1969 C-body wagons and 1971-1973 B-body wagons - 64 3/8 inches
1970-1973 C-bodies and 1972-1974 D100 pickups - 64 3/8 inches
Spring-perch measurements between the two leaf-spring mounting pads can range from 43 inches to 47 1/2 inches. This means that the leaf spring perches to mount this differential into your car may have to be moved in or out on the axle tubes with the help of a good machine shop or welder, for correct fitment to your project.
As we said earlier, these rears are easily adaptable to numerous other manufacturers, as well. They are very popular with street rod builders as an alternative to the Ford 9-inch or the GM 12-bolt rears, because they are so easily modified to suit your performance needs.
For muscle or street machines, for instance, the 1971-1974 B-body 63-inch axle will fit quite nicely under 1970 to 1979 Ford Torino, Ranchero and 1972-1979 full-size and intermediate Fords, in addition to the 1967 Fairlane, and even 1935-1936 Plymouths.
The A-body axle, at 57 1/8 inches, is similar in length to the 1957 to 1959 Fords, the 1964 1/2 to 1966 Mustangs and the 1968 to 1970 AMC AMX, Gremlin, Hornet and Javelin. Earlier applications could include 1928-1940 Fords.
The early B-body units, at 59 1/2 inches, will fit under 1937 and 1948 Mopars, and the 1971 to 1974 Javelins and Rebels, too. Early B-body units will also adapt to 1939-1948 Chevys and 1941-1948 Fords.
E-body differentials from 1970 through 1974, measuring 61 5/8 inches, can be used in 1957 to 1972 Ford pickups and 1960 to 1964 Ford full-size cars.
The 1965 to 1969 Imperial rears, at 62 inches wide, can be adapted to 1955 to 1959 Chevy pickups or 1954-1956 Ford pickups. They could also be considered for 1970 to 1981 F-bodies and 1968 to 1972 A-body GM models as well, being only 1/-inch narrower than the stock GM differentials.
Should you find a non-limited-slip rear end, or one with a smaller ratio than desired, numerous drivetrain specialty suppliers and ring-and-pinion shops can sell you replacement gears. Gear ratios available range from 2.76 to 5.57, depending on the case number and the aftermarket ring and pinion supplier you use. Yukon, Richmond Gear and West Coast Differential all offer different combinations. Posi kits and locker assemblies can also be purchased to upgrade to the Sure-Grip differential. Borg-Warner sold the production rights to their Spin-Resistant unit to Auburn Gear Company quite a few years ago.
Dana Power-Lok clutch assemblies are still available through Mopar Performance outlets, as well. Adapter u-joints are also available to convert your existing driveshaft to fit the yoke on the Mopar 8 3/4-inch rear. Additionally, aftermarket suppliers sell new spring perches if they're needed for those cases in which the leaf springs don't match up and the perches have to be relocated.
Checking into pricing with some of our sources, we find that they range from $1,750 for a complete new assembly, including brakes, to around $1,000 for a reconditioned unit. Used Sure-Grip units from a salvage yard will probably run you in the $375-$500 range, but most of the units not yet bought up are from trucks, and thus too wide for most passenger-car applications.
Some recent classified ads for 489 cases have asked as much as $400 for just the Sure-Grip dropout carrier. But equipped with some of the information we have given you in this article, you should be able to visit a swap meet or a Mopar-friendly salvage yard, with your tape measure and micrometer in hand, and find a unit that will work for your particular project for less than $600.
Keep in mind that all 8 3/4-inchers aren't Sure-Grips, but if you find one that matches in terms of spring perch location and/or wheel track width, it can be cheaper in both time and effort to buy that assembly and upgrade it than to search for a 489 case with 4.11 gears already installed for your '72 Challenger. Used non-Sure-Grip rears are abundant, and sell for as little as $100 to $200.
