Richard,
Try this:
Export a Reference Movie from FCP using the timeline attributes. Pull the Reference Movie into QT Pro.
File/Export/Movie to QT Movie
In your Export Size Settings: Dimensions/640×480 VGA
Select Preserve Aspect Ratio Using: Letterbox
You can also try Exporting from FCP using Export/Using QT Conversion. Set the dimensions and the Letterbox options as noted above.
-paul.