It sounds like you are doing a 16:9 aspect ratio I do not have much experience there, but I do know how to work around the problem. You are already on the right there with a different sequence. Maybe try and cut in the 720×480 sequence then when it is time to export, export it with a custom frame size that you had in your orignal timemine. I know this is frustrating and probably not the most ideal situation, but you will be able to see the render in the 720×480 mode, and the final will be in the frame size you need.
Geist