704/384 = 1.83, so it's not far off 1:1.85. (The aspect ratio of XviDs could be out by up to 3% because of the requirement to use mod16 resolutions.)
The second calculation is correct if the AR is supposed to be 1:1.83.
If it is supposed to be 1:1.85, then Lanczos4Resize(720,473).Addborders(0,3,0,4) would be correct.
Personally I would keep it simple and just use Lanczos4Resize(720,480) - because I have a pragmatic "that's close enough" approach to such calculations!