Factory Rear Sway Bar

turn both of those plates 90 degrees in the direction of the red arrow. those plates should be sandwiched between the leaf springs and the spring perches on your rear end. The springs will run like my horrid scribble in blue. Finally, one of your axle u-bolts goes through the two holes marked in green.

I notice that there's a ghost of the spring perches rusted into the plates on the side facing you in the pictures. I'd wager that side goes against the spring perches, and the side that faces your duster's bumper in the pics goes against the leaf spring.

The a-body sway bar mounts above the axle if I'm not mistaken, and the part that bolts to the framerail sould mount forward of the axle. That should give you the info you need to put it in if it's right and will fit. It shouldn't matter between 8 3/4 and 8 1/4, since the spring perches are in the same place on both axles. Though, if the u-bolts don't fit, I'd consider the slotted plates posted above, since they might be different. (or you could woggle the holes out.) And I concur, I believe a b-body has wider framerails, since it has wider spring perches on axles.

Though I've never dealt with this, it's just what I'd try.