You could use a Frame instead of a Text Box. Frames can be set for particular fixed locations on the page.
Frames and Textboxes in Microsoft Word

You could use Content Controls instead of a Text Box.
Create a Form Using Word Content Controls by Greg Maxey

Use Plain Text Content Controls set to not accept a carriage return and surrounded by a Group Content Control.

See also Setting Up Letter Templates - Ribbon by Suzanne Barnhill, MVP.