I am looking for a definitive answer to this question: is the smaller of the two lines that run from the fuel pump to the gas tank along the passenger side frame rails a return line or a vent line, or neither? Thanks!

My guess is return line.  The vent line goes from the top driver side of the tank down the driver side frame to the charcoal canister.

Yup...smaller line is fuel return from the fuel pump, or filter, depending on the year. Not sure if the '72 still have the return line coming off the fuel filter or not. 

Joel Adams
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Thanks guys, I tend to agree with the RETURN line answer for most of the reasons stated about there being vent lines on the DRIVER side that connect to the cannister (which is no longer there in my car). Now I need a C3 physicist to explain how and why an engine that burns fuel needs a method to recirculate it! 

I guess just a pressure relief for the pump when the bowls are full.

The LT-1 did not have a return line.

Quadrajet vs Holley maybe?