What do you think about Plum Crazy Purple on 68 Barracuda?

It's your car. Paint it any color you like.

The 68 color chart shows a color like B5 blue and another that is sort of a cinnamon/copper color that would look good with a black interior. Those are my recommendations (The copper color is used on the cover of one of the Barracuda sales brochures for 68. Looks great to me.)