Another way of doing it would be to just get your drawing area to the correct aspect ratio (1.407 in your case) and then use the "Save current window size" in the sketchup workspace preferences.
To get your aspect ratio just draw a rectangle 760x540 (the units don't matter) and then zoom it to get one axis or the other to fill your screen. Then shrink the window to fit the other axis of the rectangle. Your viewport will now be pretty well at the correct aspect ratio.
When you export a 2D image it might be larger than you need in pixel count but you can now resize it easily in photoshop (or similar) without worrying about the aspect ratio changing.
I use this method to make sure that all my files are setup at 16:9 so when I render them in Maxwell they come out the correct shape for my workflow.
Good luck,
James.