Photoshop :: Keep Guides Centered When Resizing Canvas

I'm working on a 960 grid layout, with all the vertical guides showing the grid.
 
I want to increase my canvas width from 960 to 1200, keeping the artwork centred, so in the canvas size dialog box I select the center anchor. Unfortunately this keeps the artwork centred but the guides always move left, as if they're anchored to the left of the canvas.  This means the guides are no longer in position.
 
Is there any way to add more canvas to both sides while keeping the guides where they are relative to the artwork?