Browns of two rivers of wisconsin has quite a few aftermarket patch panels for older cars. http://cooltruckparts.com/195.aspx Believe they are crosscanada parts. Not sure how they fit, think the only thing I've used of theres was a ford truck fender many years ago, but that fit alright from what I remember. Just some other places selling sheetmetal for older cars are mill supply. Sorry can't help you more with how quality and fit may be. If no one hear has bought any and has suggestions, maybe try some forums for the plymouth duster as maybe more people with that particular car will probably be present.
i bought "yearone" panels for 72 dart, the body lines were off and the panel was imported and made to slip over the top of your old panels for a cover up fit not a real repair i tried to use it as a replacement panels not a cover up , it was tough to fit since it and then it was so thin metal it was horriable to weld and i have many years of welding exp.