15X12 ON A DUSTER
Shouldn't be a width issue. On a Duster the stock wheel tub to quarter lip measurement is about 13", you can usually get a little more than that with a quarter lip trim. With a full 3" relocation and mini-tub you should be looking at almost 16" of room for tires from the frame to the quarter lip. The advertised height on those tires is 28.3" though, so you'll definitely have to center the axle up in the wheel houses, which usually means moving the axle back about a 1/2". Some of the springs out there do that anyway.
Those 15x12's you want can be had with up to 8" of backspace too, so, you should actually be able to run them on an A-body 8 3/4 with BBP axles, no narrowing needed. They'd be a little tighter on the quarters than they would be to the springs, but you'd be between a 1/2" and 3/4" of being centered. So, pretty well centered even with the stock width axle if you went 15x12 and an 8" backspace. Now, if you want that "deep dish" rim look you'd need to shorten the axle and run less backspace.
Only observation I would make is that's a lot of tire and a pretty soft compound, so, if you're going to do all of that work you might want to consider starting with a Dana 60. Especially if this is a car that will see lots of 1/4 mile trips. For a street only car the 8 3/4 should be ok, but start abusing it with those big soft tires and it might not hold up as long as you'd like.
Couple of relevant threads
Mini tub with spring location on 70 duster
This one belongs to @Marcohotrod, 15x12's with 7.5" of backspace and an 8 3/4 narrowed 1.25" per side
LOOKING FOR PICTURES MINI TUB DUSTERS | Unlawfl's Race & Engine Tech | Moparts Forums
This one has a stock width 8.75 with Dr. Diff axles and 15x10.25 wheels with a 7.5 back space and 325 drag radials
Shouldn't be a width issue. On a Duster the stock wheel tub to quarter lip measurement is about 13", you can usually get a little more than that with a quarter lip trim. With a full 3" relocation and mini-tub you should be looking at almost 16" of room for tires from the frame to the quarter lip. The advertised height on those tires is 28.3" though, so you'll definitely have to center the axle up in the wheel houses, which usually means moving the axle back about a 1/2". Some of the springs out there do that anyway.
Those 15x12's you want can be had with up to 8" of backspace too, so, you should actually be able to run them on an A-body 8 3/4 with BBP axles, no narrowing needed. They'd be a little tighter on the quarters than they would be to the springs, but you'd be between a 1/2" and 3/4" of being centered. So, pretty well centered even with the stock width axle if you went 15x12 and an 8" backspace. Now, if you want that "deep dish" rim look you'd need to shorten the axle and run less backspace.
Only observation I would make is that's a lot of tire and a pretty soft compound, so, if you're going to do all of that work you might want to consider starting with a Dana 60. Especially if this is a car that will see lots of 1/4 mile trips. For a street only car the 8 3/4 should be ok, but start abusing it with those big soft tires and it might not hold up as long as you'd like.
Couple of relevant threads
Mini tub with spring location on 70 duster
This one belongs to @Marcohotrod, 15x12's with 7.5" of backspace and an 8 3/4 narrowed 1.25" per side
LOOKING FOR PICTURES MINI TUB DUSTERS | Unlawfl's Race & Engine Tech | Moparts Forums
This one has a stock width 8.75 with Dr. Diff axles and 15x10.25 wheels with a 7.5 back space and 325 drag radials
