I’ve done this and know a bit about it. When putting a small bolt pattern A body 8 3/4” diff into a car or converting your existing small bolt pattern 8.75” diff to large bolt pattern you need to:
1- Order and use new axles with LBP, doctor diff has them New with green bearings installed for $300. Make sure to measure your diff from flange to flange to ensure it is an A body axle. I think the proper measurement is 52 5/8” from flange to to flange if memory serves.
2- next, you need rear brake backing plates for any mopar that uses 10” x 2.5” rear brakes. Junkyard ones work here from cars like: 73-76 dart and duster with 10 x2.5”, newer rear wheel drive dodge diplomat’s, rwd Chrysler New Yorkers/fifth ave’s, any mopar with 10x2.5” brakes. Grab the spreader bar and e-brake lever’s too. What ever you pull the backing plates from is what you’ll order your shoes and drums for. These parts are readily available, check rock auto.
These plates compensate for the different offset others have mentioned
3- in your case you may have to change your rear rubber flex line/brake hose if you find the 7.25” diff hose too short/long. See if you can order one that would have come with 340/383/440 darts as those cars would have come with an 8.75” diff.
4- you probably need to shorten your drive shaft and perhaps have a different rear u joint set up put on the shaft dependant upon which yolk you have on your Center section of the 8.75” pumpkin.
5- seeing as you’re going heavy duty now, you may want to look at your rear leaf springs to make sure they are up to the task too.
6- your steel brake lines from the 7.25” may not work for you on the new diff, have a look at those too.
7- hard to tell from your avatar but if you have positive offset wheels on your you could order longer studs on your axles from doctor diff and use a wheel spacer to accommodate the positive offset wheels. Then you don’t have to spring re-locate. I’d do up to an inch spacer Max, Not ideal, but doable. Maybe post a picture of your wheels.
8- install all this and giver!
