Installing new gas tank questions ?

I believe its called EEC (Evaporative Emission Control), typically a California thing but might have been standard a year or two. I think it was added mid year on 71's. Hopefully someone will chime in and check my facts.
I dont see any problem capping them off as long as you have a vent. Later cars were vented up to the charcoal cannister, others had vented caps. Figure out what you have because some caps are not vented.