I've owned two original Duster 340s, a 1970 and a 1971 and it is true that the rallye dash frame, available in the 1970 or 1971 Duster 340s, is different from the standard Duster dash frame.
You will need the '70-'71 340 rallye dash frame in order to use the rallye dash cluster. You will also need the 1970-71 rallye dash cluster wiring harness which is wired differently from the standard dash cluster.
The 1972 instrument cluster used in the 1972 Duster 340 is completely different compared to the 1970-71 340 Dusters.
The 1970-71 340 Duster used the equivalent of the 1969 Barracuda rallye dash cluster, available only as standard base equipment for the 340 Duster models.
Hope this helps.
Just a thought --- but has the owner considered going with one of the aftermarket dashes i.e. Rocky Mountain Dashes, etc?
Some of the aftermarket dashes use billet - look, or engine - turned aluminum
face plates with your choice of Auto Meter, Stewart Warner or other nice aftermarket gauge packages...
These work and look better than the 1970 - 71 factory rallye dash IMHO.
