the rally dash is its own beast. the frame itself is different from the standard dash frame (and the wiring). @72bluNblu modded the dash in his duster, so have a look at his build thread for the info there-- and inspiration! it's a great build.
rally clusters show up here frequently, or you can buy new or "versions" of it from classic dash, thunder road, classic industries. i *think* dakota digital was working on or has a johnny hot **** new complete unit, but i can't find the link anywhere.
Even Blu says get a Ralley frame to go with the IP. That's what I've done, but nowhere near ready to make the swap.
Here is his thread:
So having a fleet of Mopars, I check Craigslist and classifieds and pretty much every auction site there is to keep my Challenger running and my Dart equipped with parts for the rebuild. Which is good usually, as I come up with a lot of parts and things that are hard to find or really expensive...
He get's into the Ralley swap really quick in the thread, post #18 is the start.
The whole thread would be worth reviewing. Best I know, he was one of the first to use 18" wheels on an A-Body and an invaluable source for that and torsion bar upgrades. He even converted his Duster to a Demon.