Use CSS Sculptor to create a CSS Layout that is your desired width. Then Follow the "Preparing your Dreamweaver Template for SiteAssist Professional" tutorial in the Solution recipes section of the site assist support page to convert your css scupltor layout to a site assist professional site type.
For the menu, when you remove the existing menu, do not remove the navigation div, only remove the menu list. removing the navigation div will cause the shift you are describing.