The point is not power, he is asking how high can he rev. Because it is a d series, the rod to stroke ratio is not perfect, which means you are not spreading the force evenly in both the x and y direction. One component is going to get shafted first.
Next, when you start reving high, you have to worry about the strenth of your rods. A stronger (forged) rod will help you with strength. For the top end, you have to get better cams so you actually make power past 6800, then valve springs so you dont float a valve and or have massive valve overlap, and then light weight retainors to keep roational mass down.