On my car, (a 289) the measurement from the front vertical edge of the door aperture to the edge of the radius on the cutout is 470mm.
The approximate measurement from the top of the fender to the top of the vent is 240mm, but this could be different for a 427 body as I think the fender shape is different.
Here’s a close up of CSX3022. This might help with a visual of the real thing...
Cheers,
Glen