A quick way to fix this and not have to re output another movie is to:
Open the movie in QT
Show movie Properties (cmd J)
click Visual Settings button
uncheck preserve aspect ratio
set scaled size to 860 x 480
save your movie
I think though you selected the wrong codec when you exported or your using an older version of FCP. I used this technique when I was creating “one offs” with IDVD.
Bill Lauer
KUED Post Production