Hi
Yes, POSITION will enable you to drag your resultant cropped image near to a central position, but as you have found, you now have borders. You will therefore now need to use the SCALE effect to enlarge the clip so that the borders no longer show.
If, however, you have simply cropped the bottom from your clip it will no longer have the same aspect ratio as the original. SCALE will enable you to remove the border but in doing so you may lose some of the top of the clip or the sides of the clip. It depends on how much you cropped off.
You will have to experiment a bit so you get the best effect.
Nat