I used ACL rb30et pistons in the 25 NEO/30 i built, it ended up being about 8.8:1. All r34 rb25det's are NEO's. I would read the PDF about the conversion, as it has all the answers to your questions. You need to weld the vct in the head, and weld some extra material on the front water gallery as well whilst you are at it. Then surface grind. The cas makes no difference, as you will use the cas that matches the car adn head and cams which is all NEO. What turbo are you planning on running, as that basically dictates hat internals you require.