Temp probe placement?

Maybe better in the top tank since the manifold locations are before the thermostat. No sense for the fan to turn on before the thermostat opens, and you might need to conserve your battery in case the engine stalls, like if camping. But sounds like your fan switch trips about where the thermostat opens anyway. Once open, it is direct flow from the manifold to the radiator, so should be the same temp.

Advice above to place where easiest to wire is best. I expect the switch actuates a relay. It looks like most kits have the standard 40A automotive relay. My 02 T&C has a large solid-state relay riveted to the frame in front of the radiator, so I wonder how long those little relays last, though my T&C relay drives dual fans. If I roll my own electric fan, I may try to get one of those new relays from a junkyard.