The really scarce OEM item in the electrical control circuit is the relay, but a regular Bosch-type relay can be substituted if the circuit is wired properly. A spring-roller Micro-Switch can be used for the kickdown with a little creative bracketing. The normally-closed contacts on the relay momentarily short out the ignition to unload the drivetrain and allow the OD to disengage.

The Chevy OD engages automatically anytime the governor contacts close, (about 30 MPH) unless the OD is mechanically locked out with the cable.
Jerry

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