No, that's not what he needs.

For progressive sources, a widescreen 16:9 to letterboxed 4:3 conversion is easily implemented by an AviSynth script thus:

spline16resize(720, 360)
addborders(0, 60, 0, 60)

Interlaced sources (your Harrison Ford/Conan O'Brien video will most likely fall into this category) are more difficult:

bob()
spline16resize(720, 360)
separatefields()
selectevery(4,0,3)
weave()
addborders(0, 60, 0, 60)

Replacing the dumb Bob() with a more intelligent adaptive bobber may give you better quality.

If you need any more info I suggest you start a new thread in the How-To's and Technical Discussions forum.