The C6-R heads have 72-74 cc chambers. The D2-AA heads have 68-71 cc chambers. The range of the numbers is given for casting differences between batches. Most heads I've cc'd have been to the larger side spec wise. Also add in if the heads have been milled etc.... You can get thick or thin head gaskets to help dial things in. But you really need to know what your working with. Performance wise the C6 heads are better. Economy wise the D2's get the nod. What's the plan for the rest of the engine? No sense porting C6's and using bigger valves if the intake and exhaust are going to remain stock and be a cork. It's kind of a waste really. JMO.