Copyright the original 1975 *and* copyright your new version or arrangement 2003. Just keep both versions at hand for verification, if needed. It's mostly a matter of staking your claim, or claims in this case, so the earlier the date, the better. But your new 'version' need only be different in your own mind, as far as your newer copyright. If you want to make it official, document your changes, even if only a few notes or words different, which is likely to be the case anyway.
