I want to swap out the 7 1/4 rear end in my Duster for an 8 3/4. What I would like to know is, in order to put larger tires/wheels on the car (275-60-15 on 15 x 8 wheels with 4 1/4 BS), would an 8 3/4 from another body style be a better choice for the swap than a stock width A-body 8 3/4?

the A body 8&3/4 would be the narrowest & be a direct fit as far as pads/pinion angle but it'd have SBP axles & I'm assuming your Duster 7&1/4 is BBP & there'd be the extra weight of the 8&3/4 but I'm assuming this is going to be a performance build so yes the 8&3/4 would be needed & that you are also going to purchase BBP axles. I'm sure someone who has a Duster with an 8&3/4 will post what size wheels/tires will fit (iirc they have a large wheelwell flare).

A '68-70 b-body 8 3/4 is easier to find and cheaper than a a- body rear. They are roughly 1 1/2" longer and 4 1/2 bolt pattern. Dusters have big wheel openings so tire clearance shouldn't be an issue. Note you will need to move spring perches on tubes. Although some do spread springs to fit. Shock plates and u bolts for 8 3/4 rear are also bigger. Driveshaft needs to be shortened.

275/60-15 on a 15x8 with 4.25 BS will not fit well in the wheelhouse of a duster with a B body rear ( unless you have the car jacked way up in the rear). Either use a different offset wheel or you could have the housing narrowed.