Correct aspect ratio for mp4 file

[Mr.Madguy]

I have to transcode 720p video to non-standard resolution - 704x576. Problem is - video isn't shown correctly in video players. It's shown as vertical instead of horizontal. If I convert my video via ffmpeg - it's shown as 16:9, that isn't correct too, but at least doesn't look that bad. This makes me think, that there is some "Default aspect ratio" tag in mp4 meta data. How can I set it manually?