There is a way to do it in Quicktime after export. Doing it again in FCP is easier, but if you don't have that option, you can try the quicktime trick.
I went looking for a link to a tutorial I once read on this and for the life of me I can't find it. Here's the bits I remember. Keep in mind that some bits are possibly wrong, and maybe someone else can pick me up on them.
Export a freeze frame of your QT to photoshop.
Draw a matte around your picture. Yes, a matte. It will be removed in QT.
Can't remember if you need to make this matte pure black or pure white. One or the other. Maybe make both, if someone else doesn't know.
Delete the picture, leaving only the matte.
Save this picture. Not sure if the format is important. I'm guessing a png, tiff or similar would be OK.
Open QT
Go to Get Movie Properties from the Movie menu.
Select Video Track. Then next to this choose Mask.
Click the Set button and navigate to your matte picture.
This should crop your picture down to the correct ratio.
Save as a Self-Contained Movie.
I've done it beofre myself, so I can guarantee it can be done, I just can't guarantee my memory of how it's done. Try Google.