In includes/modules/sideboxes, there's a "classic" directory. This is your clue that the directory can be overridden. The directory includes/modules/pages does not have a classic subdirectory and cannot be overridden, so be sure to make a backup of (and notes about) any files you change.
The things you can change in includes/modules/custom are files from includes/modules, such as column_left.php.
Good luck,
Scott





