Which seats for my 63 dart?

The floor pan for the Dart and the Valiant is the same. The seats have different upholstry and design but they are the same seat frames and brackets. If you are looking to stay stock, these may be difficult to find. The ones you find will likely need to be reupholstered costing up to $500 for the pair. This gets spendy real quick. You may consider buying NEW seats from Summit racing. They have all sorts of low back styles that will be more comfortable and better supporting than the flat stock seats.