I performed this on my '71 Chall years ago and concur with all the above statements with one exception to what MEK-D mentioned about the light bar. At the time, I didn't even know there was a different bar so I just re-used my non-ralley bar and it worked just fine.
The illumination misalignment is negligible and, being as the bar is not really visible, the only one that will ever know is you. and possibly a concourse judge but if one of those is looking, the whole cluster is wrong anyway so a moot point.
So, if you're in a pinch and can't find or afford the ralley light bar, you can certainly get away with using your original.