I put a rallye dash into my '74 Duster as part of my Demon tribute. I didn't use the rallye dash frame, I just cut the original frame to fit the rallye cluster. It can be done, but after having done it I'm not sure I would do it again. I also did it with the dash in the car, so it was probably a little more challenging than it had to be. I did weld in a couple of the mounting brackets, but other than that all I really used was a set of tin snips and a set of flanging pliers. For the time and effort it's probably worthwhile to just buy a rallye dash frame.
You also don't need a complete rallye dash wiring harness. I used the harness in my '74. Turns out even going from a '74 Duster to a '68 Barracuda rallye cluster the wire colors remained pretty much exactly the same, the order was just changed on the connector. So the harness was left almost completely intact, I just removed the wires from the connector and changed the positions of the wires to match the layout of the rallye dash. The rallye had two connectors instead of just one, so I did have to splice the end of one of the feeds to go to both connectors.
One of the biggest issues is actually the heater controls, the direction and layout of the cable pulls changed, so getting the earlier heater controls takes a little work (the later ones don't fit the rallye dash). That and the radio mount doesn't line up. The rallye dash cluster puts the radio about 1" further toward the passenger than the standard cluster. Go figure. Didn't matter for me, but more work if you're swapping an unmodified rallye cluster in.
You can see the whole conversion in the thread on my car here, the dash conversion starts on page 1
http://www.forabodiesonly.com/mopar/showthread.php?t=198098
Stock...
Modified for the rallye dash, mounting tabs added, etc..
The modified rallye dash I restored and installed (more on that in my thread also)
Finished product...