You are correct.
Engaging or disengaging the OD prior to parking the car will have no affect on the static level of gearbox fluid.

To the original poster, 40 year old engines and gearboxes have been known to leak. It's good advice to clean the 40 years of grime and crud off the engine and gearbox and then look more carefully for leaks. They will be easier to find on a clean surface. Instead of throwing parts (and money) guessing at where it may be leaking, repair what is broken. All seals and gaskets are still available.