OK, for my class of racing, I have to use stock, unported heads. I can change valve size. I can't use Vortec heads, but can use any thing else. Right now, I have camel hump heads. I find that they can rev up to about 6000 RPM and after that, they start to fall flat though RPM does still increase, just not as much as 3000-6000. My engine is a 355. Stock intake and exhaust. What could I expect if I put some 305 heads on there with big valves? I can do machine work 1/2" above the intake seat, not sure if that would help with making a 305 head work or not. I heard 305 heads fall flat after 4000 RPM is that true? What about if they have bigger valves? The reason I'm considering 305 heads is for more compression. I heard the 601 casting is a good head, and it would bump my compression up by 1 full point. But it won't help unless they rev up to 6000 RPM (I'm hitting 6500 RPM at the end of the straight).