My father was a highway engineer in late 60's- early 70's his trucks were dark red orange and highway department was stenciled on door in either white or yellow. I do not quite remember the lettering color but it was spelled Hi-Way Dept. I know as a kid I thought for longest time that was correct way to spell highway.
The bright highway orange came out in mid 70's about the time Suburbans turned up. The suburbans only came in dark red but the pickups and trucks were bright orange. If Montana was stenciled on anything it was spelled Mont.
It would be great if you would post photos of your work.