Many, many moons ago, Athearn had the most easily available diesels in HO, but they made their hoods too fat (I'm guessing because of the motor technology then). I've got a lot of old magazines and I've come across several articles about making them scale width. Back when Athearn was the only option for some kinds of units that's what you had to do.
N scale has a good number of EMD locomotives that are not scale width either. Atlas' GP30 and GP35, and all Kato EMD locomotives (I think). Maybe more. Like Athearn back then, these are the only models available of these EMD units.
I had some spare shells so I decided to make a narrow, scale width GP35. I basically copied those old HO articles, and it's actually easier in N scale because the walkway is a separate piece.
To begin, I sawed the shell in half. The particular saw I used cut away the exact amount needed (~0.025"-0.30").
The front and back have to be completely new since the wide hood causes the end angles to be too wide. The front has a very convenient seam right behind the cab, while for the back I filed to the edge of the grills. The new front is from a GP40, while the new rear is from a GP38-2. An SD35 would have been a slightly better starting point, but I didn't want to risk my spare shell on the project.
I then cemented the two halves together, along with the new front and back.
This is what I have so far. I've added a little body putty to the middle seam, but it needs more. The shell needs new fans and exhaust, obviously, and that's the next step. This is my first major kitbash, so there's lots of little things I would do differently. The next one will be much better.
So, why go to all this trouble? First, because it's possible and I've never seen anyone do it before.
Second, though the difference is small, it is perceptible. I think this is more true of the GP30 and GP35 because they are short. Longer Kato SD units fare much better visually. To my eye, the difference is most striking around the GP35's inertial filter hatch, perhaps because it's flat.
The bigger third reason is that this is a warmup for a GP30 project, if I ever get around to it. The extra width is very noticeable around the cab. Just look at this image:http://rrpicturearchives.net/showPicture.aspx?id=2985980
Compared to the sleek and edgy styling of the prototype, the Atlas model looks a bit pudgy.
Things still to do: more body putty, new fans, walkway modifications, and narrowing the mechanism.