Since a suitable green color doesn't seem to be available for our nailhead engines, I have used Onan generator paint for mine,which seems to be a very close match to the original. I have used the POR-15 Buick Green which seems too pale to me,but right for the early nailhead engines. I have a '65 401 engine from a '65 Electra for parts and got a fresh can of paint from Onan last week. I blasted some on this parts engine. Here is a pic and see what you guys think of the color. Onan/Cummins is proud of this paint- $21 a spray can but covers well. When you are doing a proper rebuild/restoration, this Onan paint might be the way to go.