Just adding to the fray, my 1972 slant /6 Duster just got this . . . 8 1/4 - 3.21 rear w/5 on 4 1/2 bolt pattern. Unit is super heavy duty Police unit from an 78 Aspen drain is in the rear, right side.
The gears are stamped 3.21 and have the Diamond heavy factory Police casting insigna. Had to...