For what it's worth, Ford used 5/16" fuel line for 351 engines. If you install a return between the fuel pump and carb, it should have a restrictor in it, or too much fuel will return to the tank, resulting in starvation. All the fuel pumps I've seen that have a return have such restrictors. On an off topic truck with a rusty fuel tank I found that the engine will starve for fuel very quickly with the stock return line, but when I plug it the truck will continue to run for a long time after.
The Jalopy Journal
