Paul, I run 3/8 on both, the bypass sends excess back to the tank,
so at ignition on engine off ALL the fuel is going in a loop back to the tank.
If the return is restrictive compared to the supply line, the fuel pressure may be higher than with the engine running and flooding will result.
Of course it also depends on whether you want your pump running at IGN on to prime the carbs prior to fire up.
You can still have a pump disable (via
oil pressure switch etc) at stall condition, ie accident (heaven forbid).