You can either set your game to be 16:9, which would give you vertical black bars on the sides that would be hidden to the recording (just capture the center), or stick with widescreen and deal with horizontal bars on the top and bottom. If you stick with recording widescreen you can make a 1920×270 graphic (or two 1920×135) or something and use that to fill the blank space, with the widescreen recording above it.