From what I was able to tell when I was dealing with the rear in my 56, the "early" rear looks very similar to the 8¾ but is not as strong, has a different bolt pattern from pig to housing, and different axle flanges/bearings. After taking it apart, I can also tell you that it is much lighter duty.

That said, the E-body rear is near perfect width for the 56 Dodge/Plymouths, and I believe the 66-70 B-body rear is enough narrower that it'll allow for some extra tire. The perches would need to be moved inwards on either rear.

Hope that helps.