3/8" is only needed on the suction side(on big HP motors), the discharge side of the pump is best at 5/16", that's all you need.

All pumps for the most part no matter what there pumping should have a larger inlet than discharge. Our fuel pumps on our old cars are no different.

My 440 6 pak with factory 5/16" VS runs great with plenty of fuel from my mechanical pump on the motor.