What version of Nero Vision are you using when you are trying to burn this file?
a 704x288 resolution gives a aspect ratio of ~ 2.4:1.
4:3 is ~1.33:1
16:9 is ~1.77:1
As you can see, 16:9 is a better resolution for your video but may not be enough. You may still have black bars around your picture or a vertically stretched image. Have you tried changing the ratio in your Video options? (click on the more button, then video options, then go to the DVD-Video Tab)