Well, you are missing something critically important. The shape of the pixels.
A 1920X1080 frame uses square pixels. In a 720X480 4:3 frame they are rectangular in that they are taller than they are wide. In a 720X480 widescreen they are wider than they are tall.
If you export your 1920X1080 to MPEG2 (and I hope it is for a DVD because otherwise you should probably be using H.264) you should set the frame for Widescreen using a pixel aspect ratio of 1.212
You may still have black bars, just slivers really, on each side so you may want to "Scale to Fill".