Casino is most definately anamorph. The fact that you still see those black bars is that its format is 2,35:1 ("cinemascope") and not just 1,85:1 which is "bar-less" on 16:9 TVs.
Besides, if correctly mastered (i.e. black bars are really black so there's zero motion data), those black bars compress to virtually "0".
Edited to add:
Also, from your description, by making a film anamorph, you replace the highly compressable black bars with picture contents! Back in the SVCD days, you'd go exactly the opposite way to save bitrate: going from anamorph to letterboxed.
Your only option, besides true elaborate re-encoding, is to split the movie onto two DVDs. Not too expensive with today's media prices, plus you have no quality compromise and retain all extras/menus.
In that case I recommend DVDToolbox ( http://www.dvdtoolbox.de ) which is really fast and easy to configure.