Marty,
The 1950 Chevy would have been Negative ground.
Another good indicator for deciding if 6V or 12V "generator" (if you can't read whats on the tag itself) - What color is the tag? Black tags were 6V. and red tags were 12V.
If it has an alternator and not a generator, could still be either or, but most likely 12V. Rating will be stamped into the housing somewhere.
Let us know what you discover.
