Looking for an education in fuel tank venting

Hello gents, I read somewhere that a good way to increase the size of my 71 Duster fuel tank from 16 gallons, use a 69 a body tank. They are 18 gallons. Well my tank had 4 vent tubes. The 69 I found advertised at Classic has no vent. Why is the tank vented? Why is it not vented in 69? Furthermore if it isn't vented, how did they address the issue that caused reason for a vent. Or is venting just strictly an EPA, invented issue? Yes it is a mouthful. Thank you.