The original must suck air otherwise air would net get through the radiator.
In answer to where to fit the thermostat I have fitted a Kenlowe 2 speed electric fan to my Cabriolet. They make both 6 volt and 12 volt versions. The sensor fits inside the top hose, between the hose and the radiator inlet. They do come with a manual override and I fitted one myself because I only have the electric fan.
The rest of the controls and relays I have fitted in the jambonneau. See photos attached.