I would get some blank decal paper and run them off on an inkjet printer. Epson printers have the most waterfast inks. Let the decal dry for a day or so, then spray with Testors's decal fixative. If you don't have an Epson, I could do it for you.
What color are the paint code letters? If black, you can run off on clear decal paper. If white, then you have an extra degree of difficulty. You would have to match the background color (i.e., Jade Green) and print on white decal paper.
Alternatively, you could get a sheet of custom decals from someone like Stan Cedarleaf. who can print white on clear.
I have to confess that on my New Haven freight cars, I just faked the paint code triangle with some random squiggles. So far no one has called me on it. Your having to do four lines of text within a 7" triangle will make it hard to read anyway, at least for us geezers.
I do admire this attention to detail. Presumably, you are going to use the actual NYC codes (http://www.canadasouthern.com/caso/paint-codes.htm).
BRUCE