[Frantic] “a movie house that only take widescreen”
First, make sure of that assumption. If the movie house plays your movie off a DVD then the DVD player’s automatic mode may well play your 4:3 spot correctly, i.e. so that it appears in the middle of the screen, with nothing projected on the side “columns.” This is the highest quality solution.
Otherwise it gets more complicated. I haven’t done this yet in FCP so others should guide you on this, but if you are creating an NTSC flavor “16:9” DVD-R, you will be making an anamorphic 720×480 pixel image, either with black columns on left and right, or blowing up your 4:3 to fill the wider frame.
— Bob C