Top of the transmission cover 10 o'clock position. It has one wire and the housing is grounded to the cover with a star washer on the underside of the cover. Ground is returned through the bolts that hold the cover to the transmission case. This is the start (safety) switch, or is designed to be. Power is applied to the solenoid energizing coil via an on-off key switch but the tranny safety switch has to be pushed down to make the ground connection completing the energizing circuit.
That tractor should have a 4 terminal starter solenoid, definitely not a 3....2 large and 1 small. Neither of the small terminals shall connect to tractor chassis (ground) within the solenoid. If it does, the safety function is defeated and to start you would have to have a rotating 3 position switch....off on start.
I don't remember which shift lever, gear or hi lo lever or if both have slots in them to allow you to push down on the start switch, but one or the other will prevent the starter button from depressing if the tractor gearing is such that the tractor has the engine connected to the rear wheels allowing for movement when the engine starts.
In 65 a rotating 3 positon key switch was used and the 4 terminal solenoid and transmission were still used, but the safety switch was just that and the position of the shift/range levers determined if the switch was closed or not allowing the key switch to start the tractor in the start position.
Both my Fords are diesels.