To find it, you'll need to:
1. Identify your transmission type. This information might be on a sticker under the hood or in your owner's manual.
2. Locate the transmission. It's the large unit at the rear of the engine.
3. Look for a small, usually vacuum-operated device. It will likely have a vacuum line connected to it and possibly a small electrical connector (depending on the year and specific model). It often has a single adjustment screw.
4. Consult a repair manual specific to your 1968 Plymouth Fury II and its transmission. This is the best way to pinpoint the exact location and understand its connection points. A diagram will be invaluable.
Without knowing the precise transmission installed in your specific vehicle, providing more precise instructions is impossible. A shop manual is your best bet for accurate location information.