Without a vent the tank will build pressure when filling and may want to kick gas out the filler. Or, when running, the pump may try to collapse the tank in. It could hurt performance if the pump is trying to collapse the tank, but that would be in extreme cases. Vents on the 72 were part of the evaporative control system (ECS) and thatÂ’s why the vent goes to the charcoal canister in the engine compartment. The canister had a purge that connected to the carb to pull fumes into carb (closed system) to reduce emission. But vents have been used for years for the reason stated above, some more elaborate than others.
I doubt the open vent has anything to do with gas mileage. These cars have small tanks (15 gals), mine doesn't take long to hit "E" with a 408 stroker! You could cut the vent line down by the frame in the engine compartment and run a combination of rubber hose and new small ID hard line to a place up toward the top of the radiator cross brace or even inside the top of the front fender out of view. But make sure you put a small filter of some type on the end of the line to keep junk out of the vent. I imagine on hot days after a long drive you've been hearing a hissing sound coming from the open vent line. No problem, but annoying. By the way, the picture shows that your gas cap is vented, but I would still fix the vent line.