spring perches need to be moved a 1/2 on each side to make up the 1 in difference.... a body 43 inches apart b body 44 in apart ill post pics later this week when i install my 68 b body rear in my duster and let you know how it fits
1/2 inch is not a lot of difference. I have had some luck having a friend pull the leaf spring outward while I do the same. It is sometimes possible to get the centering pins to drop on by doing this. Once the u-bolts are tight, they aren't going anywhere!
The B body rear is 3 1/2 inches wider than an A body rear. That's about 1 3/4 inches extra width on each side. With a large wheel opening (like on a Duster or Demon) and using negative offset wheels (spaced more to the back) It is possible to bolt in and run the 66-70 B body rear just as it sits.