The bigger return line helps cool the fuel pump and the fuel, the early GTO's had a large filter right up near the thermostat with a 1/4" return on it. They wanted the coolest fuel possible right at the carburetor. A small line will bleed off the pressure after shut down, but doesn't really help keep the fuel cool. If you go to the trouble of returning fuel, dump it back in the tank, dumping it back into the fuel line just puts hot fuel right back where it just came from. A small bleed off line for shut down pressure can go back to the line, as it will have some time to cool before using it again.
Rerouting the fuel line away from the heat and in to the air stream of the fan can be a very effective way to solve vapor lock and pressure build up.
