HD QT to SD DVD

I have been working with an art gallery producing HD DVD's in DVD Studio Pro to play on their gallery monitors through their Mac. I always keep a Quicktime file of the final project for my archives.

The gallery is interested in producing an SD DVD of a previous project to sell with their exhibit catalogue.

What is the best way to take the HD Quicktime file and produce a SD project in DVD Studio Pro? Our preference is to letterbox.