Looks like 3/4" hole, so will have to enlarge the hole in the dash. Confirmed the pin-out in the datasheet. Also confirmed spring return is to run, not to off:
Looking at Connector Receptacle on Keyed Ignition Switch:
Connector Key on Top
6 1
5 2
4 3
Key in Off Position:
Pin Connect
(1)-(6)
(2) Open
(3) Open
(4)-(5)
Key in On Position:
Pin Connect
(1)-(3)-(4)-(5)-(6)
(2) Open
Key in Start Position:
Pin Connect
(1)-(2)-(4)-(5)-(6)
(3) Open
Pin Off On Start
(1) +24V +24V +24V
(2) Open Open +24V
(3) Open +24V Open
(4) Open +24V +24V
(5) Open +24V +24V
(6) +24V +24V +24V
Pin
(1) +24V
(2) Start
(3) Accessories
(4) Run/Ignition
(5) Run/Ignition
(6) +24V