Fuel line question

Terry,
You are correct regarding the second line going to the charcoal canister. Will I not need this line? If I do not use this, how do I vent the gas tank, or prevent vapor lock?
Second, my 318 is stock. I have no plans to race or anything. Will there be a benefit to the 3/8 line? IIRC they are the same price, so it's just getting the best parts for the job.
Thanks,
C