I managed to alter the layout of certain pages by doing the following.
Open includes/page_header.php, removing the following code
//// IM Portal blocks//if(empty($gen_simple_header)){      if(!$layout_forum_wide_flag&&$portal_config['portal_header'])      {            if(defined('HAS_DIED') || defined('IN_LOGIN')){                  $template->assign_var('FOOTER_WIDTH', 0);            } else {            if($portal_config['portal_tail'])            {                  $template->assign_block_vars('switch_portal_both', array());                  $template->assign_var('FOOTER_WIDTH', $portal_config['footer_width']);            }            else            {                  $template->assign_block_vars('switch_portal_header', array());            }            $template->set_filenames(array(                  'portal_header'             => 'portal_page_header.tpl')            );            portal_parse_blocks($portal_config['default_portal'], TRUE, 'header');            $template->assign_var('HEADER_WIDTH', $portal_config['header_width']);            $template->assign_var('PORTAL_HEADER', portal_assign_var_from_handle($template, 'portal_header'));            }      }      else if ($portal_config['portal_tail'])      {            $template->assign_block_vars('switch_portal_tail', array());            $template->assign_var('FOOTER_WIDTH', $portal_config['footer_width']);      }}//// end of IM Portal blocks//
then saving the file as includes/adr_page_header.php
Opening includes/page_tail.php, removing the following code
//// IM Portal//if(!$layout_forum_wide_flag&&$portal_config['portal_tail']&&(!defined('HAS_DIED'))&&(!defined('IN_LOGIN'))){      $template->set_filenames(array(            'portal_tail'             => 'portal_page_tail.tpl')      );      portal_parse_blocks($portal_config['default_portal'], TRUE, 'tail');      $template->assign_var('PORTAL_TAIL', portal_assign_var_from_handle($template, 'portal_tail'));}
and saving as includes/adr_page_tail.php
Then I replaced the file page_header and page_tail names in the file(s) I wanted to change with the new filenames. Not sure if this is the best way to do it, but it seems to work.