How to set variables in an installation profile?
You should be able to set these in the .install file in your profile. I've struggled with this well, and the .install seems to be the best way to do it. 

&lt;?php 
/**
 * Implements hook_install().
 *
 * Performs actions to set up the site for this profile.
 *
 * @see system_install()
 */
function customProfile_install() {
  /*
    Other cool functions... 
   */

  variable_set('site_403','&lt;front&gt;');
  // etc....
}