8 3/4

I've dropped B-body (65-70) 8.75 rears in Dusters... Nothing to it. I use the B-body perches (lookout! I just ducked a rotten tomato!), all you have to do is oval the hole or drill a new one right next to the original and set it in.
In fact, I made a video of the last swap on one of my current Duster's.
B-body 8.75 Rear End Install In A Mopar Duster - YouTube