Annie,
Don’t know exactly how to do this in FCP, but in compositing apps, like Combustion etc. which I’m much more versed in, here’s what you’d do.
Duplicate your video layer on another track. Create a soft edge mask on the top layer around the stuff you don’t like, and invert so that everything else is visible except the stuff you don’t want. Then, move the BG layer around until the parts of that layer that you like show through. Bing, you’re done. I haven’t done much with masks in FCP, so I’m not sure exactly how the filters work, but I’m sure there must be a rectangular mask that’s pretty simple to work with.
Let me know you got it…
DRW