I have found lacquer thinner works real well cleaning the bleed through on white letter tires. A bit tedious to scrub each letter but it works.
Another option is let just 'em bleed.
Please excuse the non-Cobra pic, but this is my BOSS 302. I painted stencil lettering on the tires and let them weather. Looks nice if you like the "I drive it hard" look. You can scrub the paint off and repaint when they get too nasty looking.
