Cacao nibs would be better carb wise than 70% choc
Taste wise cacoa nibs are quite bitter - like a very very dark chocolate - so 90-100% chocolate would be closer than 70%
Ive not cooked with chocolate much but have found that cacoa nibs dont melt as much as chocolate - Is any cooking involved in making the bars? - if so are the cacoa nibs added before or after cooking?
Try it and see - if you dont like it with 70% choc or if it puts your sugars up too much, then see if using cacao nibs makes a difference