I don't think you need to reduce the breaker to 40 amps. The 8g wire is either protected by a 40 amp breaker, or is in the section that is not protected except by the converter and battery's breaker. The 50 amp breaker is only protecting 6g wire.
The wire from my PD4655V does not have a breaker between the converter and 12v distribution board, but the wires to the battery come off that board, not off of a juncture between the board and the converter. The wires from the converter to the 12v board are a home run (aka a straight shot).