I followed your directions used Interpret Footage to change Source to 1.3333:
Type: QuickTime Movie
File Size: 23.6 MB
Image Size: 960 x 720
Frame Rate: 29.97
Source Audio Format: 48000 Hz – 16 bit – Stereo
Project Audio Format: 48000 Hz – 32 bit floating point – Stereo
Total Duration: 00:00:43:18
Average Data Rate: 552 KB / second
Pixel Aspect Ratio: 1.0 (1.3333)
…and the Sequence setting is:
Sequence 02
1280 x 720
00;00;43;18, 29.97 fps
48000 Hz – Stereo
…and the results were both good and bad. The good was that the sides didn’t pinch in automatically. The bad was that the picture is stretched:

