An SM319 from a '55-'59 Truck or '55-'57 car are a direct bolt in. If you get one from a '55 Truck w/6 cylinder, the side plate will need to be swapped with either a later one, or one from a '55 V8 truck or any Passenger car version.
Like Cletus, I have one in my '55, and easily run 75 mph @2200 rpm.
