720 X 486 is most often 4:3 picture or frame ratio with a D1 (.9) pixel aspect ratio. It can also be 16:9 with a D1 Widescreen (1.2) pixel aspect ratio. Some folks also mistakenly create original Illustrator or Photoshop artwork at this size which confuses AE because it's square pixels.
If you footage is 4:3 frame ratio then you'll have to push in to fill a 16:9 D1 frame. The same goes for any square pixel footage created at this size but intended for widescreen. The important thing in both these cases is that you interpret the footage correctly. if the footage is D1 interpret it as D1, if it's square pixels (could be but not likely) interpret it as square pixels even though AE is going to think it's D1.
The last thing you should do is try to make it fit by forcing it into a PAR that it wasn't originally. A good check is to make sure that the x and y scale values are always equal and the footage looks normal with PAR correction turned on in your monitor. If you try to fix things by selecting the wrong par your footage will be distorted.