Baskin, I see in the other thread you have a Sony W6 series TV. They are kind of known to be picky with signals and how it treats them.
I can only think of three more things to try:
1. adjust H.Active to 704 instead of 640 (this should still look too narrow, but perhaps improve it a bit because it should be a valid signal that the TV can understand).
2. Use horizontal zoom controls (if they exist on this TV).
3. Increase H.Samplerate as Steo suggested. This will make the image wider, and you’ll get an oversampled rather than undersampled image. However, it’s very likely your TV wont accept it, given it’s track record…
You are correct that 858 samplerate should be used ideally, but I’m not convinced it’s very noticeable at normal viewing distance to use 800, and for sure less distracting than an incorrect aspect ratio.