Purney,
The first check would be the most obvious...the battery. More clues might be the presence of a ballast resistor on the firewall, although this isn't 100%. The coil might have its operating voltage stamped on it and if you are industrious, the headlamp or a taillight would have a telltale number on it.
The '58 was a 12 volt system and its starter would probably not work with a 6 volt system...the '51 starter could work with a 12 volt system. The battery will be the place to start...look for more than three caps on the cells or put a voltmeter on it.
EDIT. To answer your original question, no, they would not have to change to 12 volts.
Stuart
