70 Dart, how to change regular dash to Rallye dash?

The windshield needs to come out to properly change the dash as the screws that fasten the top edge of the dash frame are underneath the windshield rubber. Also, as has been said you will also have to swap your VIN onto the new frame.....

Have you run the VIN number? Does the dash VIN match the paperwork, rad cradle and or trunk stamping?

If you want to share the VIN there are many here who will decode it for you.