Inserting bars will waste bits for en/decoding. You should use the aspect tag in the MP4 header via mplayer's -lavcopts aspect=16/9 (or 4/3, or autoaspect). If you can't get correct 4/3 or 16/9 then use -vf scale to adjust the vertical size of the content so it doesn't appear out of ratio (if your source is 2.35:1, 1.88:1, etc.). At 320x and 400x, a little chopping is not going to cost you any quality.