Light Pimp is right. On a 73 it's not a real big deal. Technically, the metal structure under the DASHPAD is part of the chassis. Usually I change the whole thing and just swap the tag, and secure it with rivits. Incidentally, and technically, you are supposed to do the swap in the presence of a Federal Authority. Yeah, like you can schedule that. Again, Light Pimp gives good advice, stating that your local Highway Patrol or VIN Verifier probably can help you out. It's not like you're trying to do anything nefarious, and the Junkyard is SUPPOSED to supply you with a reciept showing the VIN of the vehicle you bought the part from, so a DMV audit trail is established when moving identified parts. All that said: what they don't know won't hurt them. This can be as complicated or as easy as you want it to be...