A few things need to happen here:
a. Your 4x3 picture (the H264 clip) needs to conform to 16x9 picture.
b. Your 16x9 picture needs to conform to anamorphic DV NTSC.
Also, is there any chance that the H264 clip was exported from a source clip with better compression (you're taking a hit just for using H264 instead of Apple ProRes 422, Animation or Photo-JPEG at best). If so, use that instead.
It might be easiest to create a custom 16x9 Sequence with Square pixels at 640x360 with ProRes compression. Add your 640x480 and nudge it up or down if needed. Also, apply the Deinterlace filter if needed. (If it's 480i30, do this. If it's 480p30, skip it).
Add the custom 640x360 Sequence to your DV NTSC Sequence, adjusting the Scale and the Distort values as needed to get the picture to fit tha 720x480 anamorphic frame size. You'll have to open the nested Sequence as a clip in the Viewer to make these adjustments (control click the nested Sequence for this option) under the Motion tab.
-Warren