I don't think there is a horsepower advantage for a 318 in '71 to a 318 in, lets say, '74. In general, the '70-'72 Duster are lighter, but not if we are talking the "feather lite" Dusters in '76. Also, options make a difference. No performance advantage in the transmissions, and rear ends are just a matter of which gearing. Of course, the early year Dusters could be optioned with a 8.75 rear vs 8.25 in the later years. Both early and late years rolled out millions of 7.25 rears in the Duster. Obviously, the newer ones had cats.