Place preface bottom region over first sidebar instead of second

I'll post the solution provided by Sheena_d at IRC Channel which has been both helpful and useful. Hopefully it will help others in the future, as much as it has helped me:

1. First step is to move the sidebar first region. Copy page.tpl.php from Fusion core into your theme, or edit the page.tpl.php file already in your theme.

//Currently, sidebar first is printed out in this context:

       <div id="main-wrapper" class="main-wrapper full-width">
            <div id="main" class="main row <?php print $grid_width; ?>">
              <div id="main-inner" class="main-inner inner clearfix">
                <?php print theme('grid_row', $sidebar_first, 'sidebar-first', 'nested', $sidebar_first_width); ?>
     
                <!-- main group: width = grid_width - sidebar_first_width -->
                <div id="main-group" class="main-group row nested <?php print $main_group_width; ?>">
                  <div id="main-group-inner" class="main-group-inner inner">
                    <?php print theme('grid_row', $preface_bottom, 'preface-bottom', 'nested'); ?>
                   
                    <div id="main-content" class="main-content row nested">
                      <div id="main-content-inner" class="main-content-inner inner">
     

//Move the line of code that prints sidebar-first, so that the chunk of code is now organized like this:

          <div id="main-wrapper" class="main-wrapper full-width">
            <div id="main" class="main row <?php print $grid_width; ?>">
              <div id="main-inner" class="main-inner inner clearfix">    
     
                <!-- main group: width = grid_width - sidebar_last_width -->
                <div id="main-group" class="main-group row nested <?php print $main_group_width; ?>">
                  <div id="main-group-inner" class="main-group-inner inner">
                    <?php print theme('grid_row', $preface_bottom, 'preface-bottom', 'nested'); ?>
                       
                    <div id="main-content" class="main-content row nested">
                      <div id="main-content-inner" class="main-content-inner inner">
                             <?php print theme('grid_row', $sidebar_first, 'sidebar-first', 'nested', $sidebar_first_width); ?>

2. The second step is to move the sidebar-last region.

//Currently, sidebar last is printed out in this context:

                            <?php print theme('grid_row', $content_bottom, 'content-bottom', 'nested'); ?>
                          </div><!-- /content-group-inner -->
                        </div><!-- /content-group -->
     
                        <?php print theme('grid_row', $sidebar_last, 'sidebar-last', 'nested', $sidebar_last_width); ?>
                      </div><!-- /main-content-inner -->
                    </div><!-- /main-content -->
     
                    <?php print theme('grid_row', $postscript_top, 'postscript-top', 'nested'); ?>
                  </div><!-- /main-group-inner -->
                </div><!-- /main-group -->
              </div><!-- /main-inner -->
            </div><!-- /main -->
          </div><!-- /main-wrapper -->
     

//Move the line of code that prints sidebar-last, so that the chunk of code is now organized like this:

                            <?php print theme('grid_row', $content_bottom, 'content-bottom', 'nested'); ?>
                          </div><!-- /content-group-inner -->
                        </div><!-- /content-group -->
                       
                      </div><!-- /main-content-inner -->
                    </div><!-- /main-content -->
     
                    <?php print theme('grid_row', $postscript_top, 'postscript-top', 'nested'); ?>
                  </div><!-- /main-group-inner -->
                </div><!-- /main-group -->
             <?php print theme('grid_row', $sidebar_last, 'sidebar-last', 'nested', $sidebar_last_width); ?>
             </div><!-- /main-inner -->
            </div><!-- /main -->
          </div><!-- /main-wrapper -->

3. The last step is to add the following code to your template.php file. This reverses the way the preface bottom and postscript top widths are calculated. Create a blank file in your subtheme's root folder, name it template.php and paste the following function:

    function MYTHEME_preprocess_page(&$vars) {
     
    /*Altering the way Fusion usually calculates grid widths for preface and postscript regions so that sidebar-first clears preface and postscript regions*/
      $grid_name = substr(theme_get_setting('theme_grid'), 0, 7);
      $grid_type = substr(theme_get_setting('theme_grid'), 7);
      $grid_width = (int)substr($grid_name, 4, 2);
      $sidebar_first_width = ($vars['sidebar_first']) ? theme_get_setting('sidebar_first_width') : 0;
      $sidebar_last_width = ($vars['sidebar_last']) ? theme_get_setting('sidebar_last_width') : 0;
      $vars['main_group_width'] = $grid_name . ($grid_width - $sidebar_last_width);
      // For nested elements in a fluid grid calculate % widths & add inline  
     
    }