1972 Duster factory electronic ignition harness into a 1971 Duster 340 (rallye dash)?

Do you have a pic of the back of the instrument cluster you want to use? Different circuit boards depending on model.

The Dash Bezel in your interior pic looks like a 70 but hard to be sure because of the radio cutout.

Also, 67-69 Rallye dash wiring had a bezel mounted emergency flasher switch

The dash frame in the pic is not a 68 Barracuda, Its a 67 , 69 or 70-71 . The cluster may be a 68 barracuda, if so, the 5 volt voltage limiter is in the fuel gauge and is not mounted to the circuit board.

The 68 would have had holes in the bottom of the metal frame top mount the lower dash pad. (one year only)

I'm thinking if the cluster has the correct circuit boards, you are using the factory Mopar electronic ignition, and the wiring is in place for the emergency flashers it should work. Make sure about the voltage limiter though.