Like most cars & trucks of that era it would be mounted below the cab floor on the inside of the left frame member. The frame is drilled for the mount holes and the hole in the floor will line up with the filler cap. There will also be linkage that attaches the MC to the brake pedal.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine