You can install CJ sized valves and they will be close to the same. The CJ heads exaust port is higher and wider so you'll need to port match the C6-R's to match. Also the CJ combustion chambers are a little different. They were called poor mans CJ heads beacuse you could pickup a set cheap. Install the CJ size valves, port and basicly have a set of CJ heads. Or close to anyway. G.