Any chance you have a build sheet?
I agree that it's pretty darn unlikely that just the dash was swapped. That would be unusual to say the least, but of course anything is possible.
I think it would actually be MORE likely that the car never came with one. Now, I'm not saying that I've heard of '70 340 cars that didn't have rallye dashes. But, knowing Ma Mopars track record, I would actually be less surprised that it came that way than if that was the only thing swapped.