Legal question about Antique vehicles and the seat belt laws

In WI, until the kid reaches 80# they need to be in a booster seat in the rear seat. I called the local police station. 1970’s car so no air bag to face slam them. I have installed shoulder harness in front seat with only a lap belt in back. They agreed the child would be safer in the front. I haven’t been pulled over yet to see what the officer thinks.