Something weird that I found. I went and followed with your suggestions, but the output was still stretched on the 1920×1080 workspace.
So I went back and checked the attributes of individual clips and found that they were all 1920×1080 with square pixels and no compression. I decided to check the overall sequence attributes and saw that they’re 720×480, with h.264 compression and the pixels are NTSC-ccir 601.
Why? Is this the source of my problem and how can I fix it?
David