'68 Barracuda Notch

Since the tires have already been mounted (but not driven on), I'm not sure if they'll swap them out for me or not.

Never hurts to ask, especially since you are a good and loyal paying customer and all. 275/35-18 is a very popular size. I used to work at a tire shop, we used to test fit stuff all the time. Wheels is a different story, not many places will take a wheel back that has had a tire mounted on it.