Bob,
GM switched to 12 volt in 1955 so your 58 will have a 12 volt system. You did not state what year the engine came out of, it is possible that if it was installed in an earlier vehicle it could have 6 volt components. Look to see if there is a tag on the starter & generator.
I upgrade all of my older vehicles to alternators but it is a personal choice, there are still many trucks running generators and they work fine.
Fred
