What's a good rear end to put in my Duster?

A '60's B-body 8.75 is easiest/cheapest way to go. No need to move spring plates.
Use spacers(or even stack of washers)on rear shackles so each rear of spring is
Spaced an inch or so outward- that's it! Dusters have open wheel openings so
tire clearance isn't a prob. Did you ever notice on a disc brake Duster front tire track
is wider than rear? FIXED! 4.5" bolt pattern, bigger brakes, very simple and cheap.
Me and my brother both did this on our Dusters.

Pre-71 B-body rears have perches at 44" c-c. A body rears have perches at 43" c-c.

So, you get longer bolts for the shackles and just throw some washers in there? :violent1:

You're putting a bind on your front spring eye bushings, forcing an additional side load onto the springs, and putting them on an angle front to rear. Not to mention increasing the stress on the shackle bolts. You even lose an extra 1/2" per side for tire clearance for your trouble.

Just move the perches. It's not hard. Anyone that has to share the public roads with your car will thank you.

But yes, if you move the perches a 68-70 B body rear and works pretty well in a Duster to even up the track widths. That's exactly what I run in my Duster, after I relocated the spring perches.