It is very unlikely that a TV will receive a 70Hz signal at any resolution but 640×400 or 720×400, unless its scaler is more like a monitor than a TV with plenty of VESA modes.
To stick 640×400 in a 640×480 frame, the output mode for 400p should be in passthrough, but the image is blurry like plugging the PC straight to the VGA port on the LCD monitor.
We want Line2x for crisp and clear DOS image on an LCD.
In line2x mode I had to change the active vertical resolution to 480 from 384 so my monitor reads it as 1280×864 70Hz, only then the 4:3 option is selectable in the monitor, but on 1280×720 and 1280×800 it is always Wide.
As you can understand the current implementation of 384p is definitely not designed for a DOS PCs but manageable with advanced tweaking.
Ideally, 640/720 x400 frame in 640×480 doubled to 1280×960 resolution would be great, but 960 active requires at least 1000 lines total vertical resolution which 449p (400p) does not provide.