Solved!
O.K. So the source video codec was DV
720x480
29.97 fps
format: yuv411
aspect ratio 16:9
Interlaced BFF
I started a new project and prior to importing selected my settings I set video mode to SD NTSC (tried to match it to my source but didn’t see an option for 720i at 29.92 fps)
At any rate I tried a number of export options for .mp4 and all were full 16:9 with no black bars. Interestingly enough the YouTube .mp4 format gave the best results.
Anyway, not sure which steps I did that fixed this, but I guess the lesson is to make sure you take time to set up your project first.