It might be worth checking out how much it would cost for a hot-rod pin-striping expert to paint the line on?

Otherwise possibly get some drafting film, draw the pin-striping line on that, use some low-tac spray glue to fix it to the surface, then cut out the stripe section with a scalpel. Clean off the glue in the gap, spray black, remove film, then clear off the rest of the glue before clear coating.