Some came with factory dual exhaust. My 400 did. Your exhaust system will eventually need to be replaced again anyway, so go with what you can afford and what works well now. If you are ever concerned with being completely correct for restoration, you can have a repro exhaust system created. And if your 'bird originally came with single exhaust, you may like your duals too much to change back.

Just make sure you get a good installation that gives you ground clearance as good as factory, so you don't drag the mufflers on curbs.