1/4 inch line? Probably vapor return from the fuel filter, I don't think '72s had charcoal cannisters yet (maybe California?). Best bet is check the FSM for your model.
Since you are going to EFI (which EFI, BTW? What engine/body? What does the EFI manufacturer recommend?), are you moving up to a 3/8 supply line? If so, use the factory 5/16 as your return, I wouldn't use the 1/4. JMO.
