Zen Cart draws its content via tpl_main_page.php

Inside that, it calls content for the header, then the "left column", then the "center section", then the "right column", and then the footer.

You've got ALL of that set to show up in the middle of the screen, with whatever amount of space the customer sees on the left/right based on their screen resolution.

If you want to put something in the left column and then use CSS to push it outside the main content area, go ahead. You will lose the ability to expand your center section content into the left column. If you want to put your menu code into the footer, and then reposition it up and left from there, I guess that'd work. To do this, stuff it somewhere in tpl_footer.php, and wrap the appropriate block tags around it so you can relocate it happily.

It all depends on browser compatibility. The challenge of getting all browsers to honour your positioning requests will be fun I'm sure.