1955 1957 Chevy Tri 5 BLACK OUT 7" Power Brake Booster Bail Top Master Disc Disc

This is the best deal you'll find on a quality setup like this. The booster mounts with a 4-bolt pattern. The 4 bolts are in a perfect square and are 3 3/8" apart (center to center). It can be mounted on the firewall or under the floor.