I'm static numbering them, or painting the numbers on the cab and numberboards
I don't know that you can paint on the numberboards without incurring problems like this. I'm not absolutely sure, but I think each number is placed on a part in the 3d model. There're designed for autonumbering. I'd still recommend using autonumbering for those numberboards, although the original model is using a 3 digit format and you have a 4 digit format. That could be another obstacle in this case. Could you go to 3 digits for your project, as it is fictional?
I've had issues like this when there is only one side of an object represented on a texture file. Locomotive tenders are a good example. One side is mirrored on the other side, while any lettering is on a decal.
tagsdm