South Carolina state law requires that babies ride rear-facing in a federally approved car seat until they are at least one year old and weigh at least 20 pounds. Children ages 1 to 5 who weigh 20 to 40 pounds must ride in a car seat. South Carolina law further requires that children under age 6 who weigh 40 to 80 pounds must use a booster seat, and children under age 6 may not ride in the front vehicle seat, unless all rear seating positions are being used by other children, or the vehicle does not have rear seating.
This is what the law is - the car is "grandfathered" beyond these minimum requirements legally - however, that doesn't limit reasonableness under the circumstances that you may want to do more when riding in this classic car.
Your vehicle would be "grandfathered" in as it is. However, you must comply as stated:
No, there doesn't need to be shoulder straps in this case. But you have to follow the minimum as to children.
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