Which year(s) B-body axle are you running; 65-67 or 68-70?
1968, but Dusters are pretty wide in the back. My friend has a 65 axle in his Demon and I didn't notice much difference, nothing that couldnt be made up with a small wheel spacer.
I measured an all original metal one owner 70 340 Duster yesterday for inner to top 1/4 lip inside edge and go the same exact measurement as the original 68 Barracuda coupe wheel wells I have.
Orignal metal '70 340 Duster
68 Barracuda coupe wheel well with orignal 1/4 panel lip skin still attached