The disc prop valve replaces the drum one and is mounted to the lower DS frame rail. Is yours factory or aftermarket? If you are using the 73-6 style prop and disc brakes you may have to change the line that passes across the firewall. I just bought a kit from finelines for a 73-6 duster with disc brakes. If your doing the same thing just get the kit for a 73-6 Dart with disc brakes.