if I am understanding your questions: 720/400 means 1.8 aspect ratio. so it will be a little wide than HD 1280x711 (instead of 720). If I need to upload this file to any of the video publishing services (vimeo, youtube) I would use the HD preset because if you upload a 1280x711 (or 720x400) file, you will get a little black bar anyway at the top and the bottom of your video. read here: https://support.google.com/youtube/answer/6375112
Assuming you mean 720x480 (standard definition NTSC DV), there's a composition presets you can use (NTSC DV Widescreen). The resolution remains the same, but the pixel aspect ratio changes (pixels are stretched) to make it 16:9.