7 1/4 to 8 1/4

If the shock plates aren't there it's quite easy to slot the holes on the 7.25 shock plates to accept the larger 3" u-bolts.

If I recall correctly when I did this I had to have the drive shaft shortened 1.5". If the car the axle is coming from is another Duster try and get the drive shaft too, it will save having to cut the one you have.