since there is none made for barracuda 67-69 witch would be better to use, the ones for Dusters or the ones for Darts?

The duster wheel opening is closer to the Barracuda design than the Dart design. So that is the my answer for trying to match the front. I'm not sure which style would be a closer match at the rear. If you can salvage the front or rear part of your extension, you may be able to fabricate something using pieces of new and old extensions.