need advice: 440 in a Duster
If you run a stock to mild 440 you could use the 8 1/4 it is stronger than most people realize. Mopar made plenty of big block cars in the 70's that had 8 1/4's in them (I have one of these cars). These rears also found there way into trucks and vans. The weakest link is the 27 spline axles but if you use street tires they will be weaker than the axles. You can upgrade later to the 8 3/4 but the rear end is the last thing I would change if at all.
Chuck