I'm not up to date on resizers for AviSynth, but there are differences whether you use them for enlarging or reducing (some are good for one task but not for the other). Seeing that your captured film's anamorphic squeeze is not the same as the DVD's anamorphic proportions, you must resize your video.

Yes, you are correct in your procedure (to produce a DVD). As needed, you first adjust (non-proportional reduction in this case) your film's anamorphic proportion. Just resize the image to match that of the DVD direct capture (720x480) and it'll be right. Note it will still be in an anamorphic state at that point. Next, you set the video/authoring's anamorphic flag so that the DVD player will unsqueeze it to a 16:9 display or letterbox it to a 4:3 display. That's it.

DVD has a simple procedure for dealing with TV displays:

video . . . anamorphic flag . . 4:3 TV . . 16:9 TV . . displayed
720x480 . . . . . . . . . . . . . . . . . *. . . . . . . . . . . . . . . . . direct
720x480 . . . . . . . . . . . . . . . . . . . . . . . . . *. . . . . . . . . direct + sidebars
720x480 . . *. . . . . . . . . . . . . . *. . . . . . . . . . . . . . . . . expand + letterbox
720x480 . . *. . . . . . . . . . . . . . . . . . . . . . *. . . . . . . . . expand

In practice, there are 3 display proportions that most movies follow:

Academy (4:3) = 1.33 aspect ratio
Widescreen (16:9) = 1.78 aspect ratio
Cinemascope (2.35:1) = 2.35 aspect ratio
(note: Cinemascope is a brand name; it represents all the other extra-widescreen formats)

The DVD standard is built to handle the first two and can accommodate the third (or any proportion-variation thereof) if partial letterboxing is pre-added to the video.

So, the Academy filmed Star Trek TV series would display like this:

The Widescreen filmed Silent Running would display like this:

And the Cinemascope filmed Star Wars would display like this:

NOTE: DVD media (720x480) raw-displayed on a computer screen APPEARS "fatter" than it does on a TV display. However, software DVD players generally reduce the viewing width (and screen captures) to 88.89% so it LOOKS correct. Activation of this aspect ratio correction usually can be toggled in the settings. So, too, the above examples have been reduced 88.89%, just to look better.