Those were illegal here when I had my '41 Ford, so I put lakes plugs on it. Extensions from the head pipes that came out under the doors with metal caps bolted on for street use. I had the caps on with 1 bolt and self locking nuts, and they were strategically placed so that the could be kicked shut when getting out of the car.
I saw a photo not long ago of a police motorcycle garage around 1918. There was a huge sign that said, "KEEP YOUR MUFFLER CLOSED!"
