Well, the problem is that you will either need to use letterboxing (“those black bars”), or lose the left and right edges of your HD composition. I would recommend the 2nd option although it may take some reworking if you had necessary elements at the left and right edges.
Create a new NTSC DV 720×480 composition, drag your finished HD composition into this SD composition. Then with the HD comp selected in the SD comp, go to Layer > Transform > Fit To Comp Height.
It should look pretty good since you’re reducing the resolution, but you can try enabling the “Collapse Transformations/Continuously Rasterize” switch on the HD comp inside the SD comp and see if you like the results more.
The nice thing about this method is that you don’t lose your HD comp, so if your boss ever changes his mind (happens all the time) and wants an HD version, all you have to do is render the HD comp.
Darby Edelen
Designer
Left Coast Digital
Santa Cruz, CA