Baking chocolate question?

I have a recipe for a chocolate souffle cake that calls for 6 oz. of bittersweet dark chocolate. I have 1 Ghirardelli bittersweet baking bar, but that is only 4 oz. I have 2 other forms of chocolate on hand: some Lindt 70% dark chocolate bars, and unsweetened baking cocoa. Which would work best, if at all? Thanks.