4:3 is probably going to be the easiest. You could do a 16:9 standard def, but it would be a ton more work. I'm already replying and can't remember what you said you were editing in FCP 5 or 6. In 6 you can just set up a sequence in 4:3 SD and drop in the assets. For the HD footage you can certainly shrink it down and use your key frames to scroll around the shot. If you are in FCP you can do the same, but you have to render when you drop in the HD footage. Make sense?
Good luck!
keytohwy