My bad I see your title says 68 Dart. I had to buy HT quarters for my 68 Sedan Dart and modify them to work. I thought maybe this might be your case. But Yes if it is a hardtop,send them back.
At that price I'm assuming you have a quarter panel skin and not a factory style replacement quarter panel. The quarter skin is trim to fit while the factory replacement style you drill the spot welds and mount as the factory did during assembly.