If it fits a V shaped Chevy engine or any other GM passenger car / light truck it will fit on a 250. The aftermarket 140 amp jobs that use the Delco case are a physical exact replacement. If you are running an ammeter the range needs to be enough to indicate all the amps that passes through it. A voltmeter doesn't care as it only displays the voltage output. To get the full benefit of that alternator's amp output you need to run at least an 8 gauge wire from the Batt terminal of the alt to the battery. Wire is like pipe, the smaller it is the less volume that can pass through it and amperage is the volume in an electrical circuit. Amperage does the work, voltage only gets it there.
Mike