Center all vertically and horizontally?

I'm trying to self-publish a book using CreatSpace on Amazon.  They want an extra 1/8" around all margins of a 6x9" book (actually more on the width) to allow for bleed.  I have to generate a PDF.  When I changed the page size it kept the type in the same spot, and made the pages jump back and forth a little in the PDF.  It would be good if they stayed in the same place.  How can I center ALL the artwork -- including the headers on the master page -- both vertically and horizontally to fill a particular page size?  Thanks for any advice.