What engine does your '51 have? Original 216 or newer model 235? If it is original 216 then you would want to get points for '51 216 (points don't care whether they switch 6 or 12 volts). 12V coil from 55-59 is fine. Anything earlier than '55 would be a 6 volt coil.

Do you know how to identify the different engines?

PRW