2 possiblilites for the symptom.
The switch has bad or burnt contacts inside and may need rebuilding.
Secondly try checking that the switch casing has a good ground as well as the motor.
I'll put my money on a bad switch as it is 40 years old and needs servicing.
Was the 3 speed motor rebuilt or checked to make sure it functions properly?
Are you sure you have good contact points with all the terminal connectors?
The symptom can also mean that your blue wire is not getting the signal to send the motor in reverse so that it can park.
Have the wires in the connector to the dash switch been fiddled with?
The linkage for 2 and 3 speed motors is the same on 70 B bodies.
You do have to change the cam arm to a 273 ending part number.See pic.
You also need a tan color nylon bushing and not the black 2 speed bushing.
The wiring remains the same.
If you need my services please contact me.
Thanks
Jules