As others have mentioned. That figure is easily achievable with stock heads (even the small valve) without any work. I've got J heads on my 340. There's a local headshop/raceshop that can do all out porting, cleanup, valve work, etc. For about 1000. That should get me in the 260+ CFM flow area. Good enough for 550 (potentially more) horsepower. If I want it in the future.