Sorry about the delay in responding.
The small engine can use all the velocity is can get with the small cam. The larger open chambered heads reduce compresion and will require more timing to run best. The larger ports will have less velocity to atomize the air and fuel charge. This means bigger droplets of fuel in the cylinder. These bigger droplets are harder to fully ignite and burn. A full burn of the air and fuel is what everyone is looking for or your just wasteing the gas.
Ported, the 318 head can still out perform a 340/360 head ported or not. It will still have a higher velocity than the bigger port heads.
Remember, the dyno may say big power, but how it acts in the car is a different story. I'm generally not a fan of anything bigger than a 650 on a 318 when it see's the street. A track 318 is another story. I don't care how much anybody screams about the bore size of the carb and how what acts like what. IF your driving the street, and it's not a crusher of a teen, you want the 318 heads and not larger than a 650 on it.
360 heads on a teen are OK and can be cheaper than porting 318 heads. You'll give up torque down low, but then again, you or the engine may not care. Things like a bigger cam, higher stall converter and gears aren't even operating in the lose area.