My duster came with 8 1/4 3.21 sure-grip. However it also had the 360 4 bbl. from the factory. I geuss one could probley get the 3.21 sure-grip for any eng. combo if the box was checked on the order form.
I don't believe mine would have came with 3.21s because the original owner was a older lady who only drove a mile to and from work. I know it didn't come with a sure grip.