If the recipe specifies the cacao percentage, it's best to stick with it, or at least use something that's close. The 70% could cause a drier result in texture, plus may be too bitter in proportion to the sugar, but that solely depends on your individual taste. Can you find Ghirardelli near you? They have two kinds of bittersweet bars, one is 60% and the other is 70%. You should be able to find the 60% easily. They also have a semi-sweet bar, which doesn't indicate the cacao percentage on the wrapping, but I would venture to say it's probably about 50 - 55%. That may be your best choice, but either the semi-sweet or 60% bittersweet should be fine.
