Adam, acording to m.p manual the steel crank was used in the 73 340 model year with the 4 speed transmission. I owned a matching # 340 73 duster and 4 speed and it did indeed have a steel crank as damper casting indicated this.
i thought i remembered hearing/reading that 4 speed cars still got the forged crank. but then again, maybe which factory built it may or may not have run out of the forged units.