I would go for shrinking the decal than modifying the model to take it. (Which is "worth" more?)
You could scan and reduce it onto "tranfer paper" it's a kind of paper that goes into most inkjet (and colour laser as long as you check it can be used) printers that you can float off the decal in water. Not sure what you would call it... stencil paper or tattoo paper?
All I know is it exists as I have some. You print and then float it off the paper in water.
PS.. not sure how "prototypical" it would be, but if you delete maybe a mm of white space between letters and gap between words by half then it might just fit
Great suggestion and I've tried it out, but I've only an inkjet printer so it seems I'm stuck using Red or Yellow lettering as nothing else stands out and that looks horrendous and so ridiculously small it's unreadable.
Maybe three good, easy options:
1. Cut a piece of .010" styrene the proper width and longer length to fit the decal, then glue in place over the existing board.
2. Make or obtain a smaller decal to fit the existing board.
3. Buy a passenger car already lettered for the C.N.
Anything else may possibly work, but probably ain't gonna' be as easy.
A smaller decal would be N scale, and I can't find any that are for passenger cars of this period, they all seem to be the newer noodle design for locomotives and freight cars
I did find some Sante-Fe cars in the UK that could work, but they're Budd Heavyweights which are too long, they won't track round my layout.
I did find the exact cars I need, (always too late) that I can use but in the US. $60 each plus postage ($40) and import tax, ($20) so that's a no-go, and anyway, I've put too much time and $ into these three cars to throw them away, (the interiors alone cost me well over $100)
I have tried using model filler on one of the cars, but with the filling and sanding I'm pretty much back to square one and having to do a re-paint, if I had realised when I started what I would need to do to get these decals to fit, it would have been so much easier as it would have been done pre-initial re-paint, next time I'll wait until I have everything I need in hand before I start a similar project.
I think the styrene method works the best in this situation
So I'm using styrene and glueing them over the top, I've got enough CN paint to re-do the car and paint 4 X 10cm long replacement boards.
Ah well, live and learn I suppose, thank you all for your help and advice, it's very much appreciated.