The Kato car has an extra service door too though...
Here's the N&W car again:
Here's the Kato Diner from Fred Klein's site, kitchen side.
Trying to swap roofs is a good idea. If that works, then you can file off the UP-esque batten strips above and below the windows. Then remove the extra service door on the Kato car and move the square window from the end of the car to position where the extra service door was.
Both cars need skirting also. But the CCS car has partial skirting at the ends already. CCS car would need the vestibule door removed and a kitchen window blocked. At least on the kitchen side of the car, the CCS body is still less work.
What I'm realizing from all of this is that N&W passenger modeling is too much work.