Each standard block should be a Record in a table. You can set up tables and relationships such as:
Clients::__pkClientID = Messages::_fkClientID
Messages::__pkMessageID = SelectedBlocks::_fkMessageID
Blocks::__pkBlockID = SelectedBlocks::_fkBlockID
This assumes that you might send more than one such message to the same client. If you will never send more than one, you don't need the messages table and can link Clients directly to SelectedBlocks.
A List view layout based on SelectedBlocks can be set to print out your message. Clicking a button can select a particular block and create a record in SelectedBlocks that is linked to that Block record. The field that contains the Block text can be set to "slide up, resize enclosing part" to produce a printed, preiviewed and/or PDF copy that removes unused space between the blocks of text.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
And as a way to personalize those blocks of text, you might find this demo file of interest: https://www.dropbox.com/s/6xw8buafjuohncu/MessageTemplateBuilder.fp7
For an example of an interface design that you might use for selecting blocks of text for a given message, see the check boxes layout in this demo file: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
These demo files can be opened from FileMaker's File Menu to get a converted copy of the file if you are using FileMaker 12 or newer.
Thanks Phil, yet again a great answer!