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Here is a
Wiring Diagram for 1954 & Early 1955 Chevrolet Hydra-Matic Auto transmission.
The Hydra-Matic Auto transmission wiring is included (dashed lines) which has a neutral safety switch.
Starter circuit is closed, when the steering column shifter linkage is in neutral.
The 1954 & Early 1955 Chevrolet Hydra-Matic Truck used a keyed on ignition switch along with a push button start switch.
In your case you would likely use a keyed spring ignition starter switch and a under dash clutch pedal switch.
Where as the starter circuit is closed, when the clutch pedal is pushed towards the floor.
Something like
THIS Clutch Pedal Position Switch
