Converting an MPG data file burned to disc to a burned "video disc" for set-top DVD players
Encore should work fine for your purposes.  When you look at the original source, what is the resolution?  Is it still 728 by 480?  Most likely the original image is using oblong pixels to make a 16:9 image with a non 16:9 resolution.  This is fairly typical and is called Anamorphic Widescreen.  The 16:9 image is compressed to a 4:3 size (720 by 480 is a typical .9 pixel compressed NTSC 4:3 image.)

When recording it to a DVD, you want to make sure that it is encoding it as an Anamorphic wide screen so that the DVD player that plays it back knows that it should scale it out to 16:9.  Your other option is to letterbox the original to a 4:3 size with black bars on the top and bottom.

I don't have Encore in front of me at the moment, but if you can't find the setting for anamorphic widescreen, I can look it up when I get home and let you know.