848 x 480 isn't an exact translation of 840x840 to a 16x9 stretch.
To think about aspect ratio, you need to consider what shape your image is now, and what shape you want it to be. If you have a 480x480 image that looks correctly proportioned on your computer screen, then it is using square pixels (ie each pixel is the same height as it is wide).
Imagine your footage showed a circle.
When you play back your footage correctly, it should still look like a circle.
If it was filmed at a different aspect ratio, then you may need to squash or stretch it to compensate for the change when you play it back.
480x480 stretched to a 16:9 aspect would become
16/9 = 1.777778
480 * 1.77778 = 853.333
So your new aspect ratio might be approximately
854 x 480.
But you need to look at it and confirm what looks right. Often with old footage, aspect ratios aren't quite right, or aren't exactly 16x9 or 14x9.
The correct way to do this in a video editing app would be to create a sequence with the settings your video should be delivered at (e.g. HD video at 16x9 with square pixels is 1920x1080), and then place the source material into this sequence, and adjust the source video until the scaling is correct, either by maths or by eye.