[Robin Probyn]"What actually is the best way to get 4-3 footage to 16-9? "
Shoot 4x3. Make the 16x9 video with black bars on the sides. This results in the highest resolution, but know that it still won't be great resolution... especially if this 16x9 picture is supposed to be HD.
[Robin Probyn]"Can I just frame the top as the top, and the bottom be cut off, rather than leaving headroom at the top and bottom."
Yeah, you can do that, too. There will be a little extra work to do, however. If you can get your editor to "fit the video so it fills the screen from side to side", you'll then probably have to shift all the clips down.