One thing to research if you are thinking of an aftermarket valve, the vacuum valve for my '57 has 4 ports, vacuum "in" from the engine, vacuum "out" for low range, vacuum "out" for high range, and exhaust "out" to the exhaust side of the system. The valve has to be able to shift the vacuum to the actuator in both positions but also has to be able to exhaust the circuit that is being de-energized.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy