I had this exact question myself when I anamophicised my star wars LD.
superman is a 2.35:1 aspect film if I remember correctly so this is what you do.
crop the black bars so you only have the film image.
if its NTSC it will be
720x 270ish
if it's PAL it will be
720x 325ish
resize it to
NTSC
720x360
PAL
720x432
then add black bars, either in avisynth or by virtualdub up to the full resolution
so NTSC
720x480
PAL
720x576
that will give you an anamorphic sized capture.
then when you convert it in something like TMPGenc to give you your M2v file for authoring, make sure it is flagged as a 16:9 file.