Identify this 8 3/4
Best thing to do is have it narrowed to a body width, send moser the axles and get them shortend and reslpined to a body width, and be done. Not very expensive either really.
Not for a Duster. I wouldn't go anywhere near A-body length for a Duster with stock or even 1/2" offset springs.
If you want to be able to rotate your tires, and you don't want the rear end to look narrower than the front, your best bet is to narrow it to 65-67 B-body specs, or 54 1/4" flange to flange. That will match the wheel offsets front to back pretty closely.
You could go with a custom width too, but the 65-67 B rear is so close to where I would want it to be for a Duster with the stock spring locations I wouldn't mess with it. That way if you ever wanted to buy aftermarket axles you could just buy them off the shelf instead of having to pay extra for a custom length.
When I switched out my 7.25 for an 8 3/4 on my '74 Duster I used a 68-70 B-body 8 3/4 rear. It's wider than the 65-67 by about 3/4", but I was still able to run 15x7" police rims with a 4.25" backspace and 225/60/15's all the way around. I was too close to the quarters to run anything wider than that, but the 65-67 rear would solve that, it wasn't much. Kind of moot anyway as it's pretty hard to run anything wider than a 235 on the front with 15" rims. And if you go to larger rims, say, 17 or 18, you'll find that the offsets needed front to back will be VERY similar, with ideal for max tire width being about a 25mm offset front and back.