Use the spots plugin:

http://wordpress.org/extend/plugins/spots/

You can use it as a widget, a shortcode, or place 'template spots' in your theme that you can assign.

You could also use custom fields in metaboxes to add edit boxes for each field

Use the spots plugin:

http://wordpress.org/extend/plugins/spots/

You can use it as a widget, a shortcode, or place 'template spots' in your theme that you can assign.

You could also use custom fields in metaboxes to add edit boxes for each field

edit:

You could always have a custom post type for each type of block, and a custom taxonomy, and display all posts in a certain term, e.g. for Page A you could have a 'page A top thirds' term and display posts in that