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I think you need to consider the operating temperature of the fan switch. For arguments sake, if your thermostat opens at 88C and your fan switch (fitted to the bottom hose) operates at 95C, would you want to wait until the coolant leaving the radiator reaches 95C before the fan cuts in? What would the temperature of the coolant leaving the engine be at this time? If the fan switch is located in the top hose the fan will cut in when the coolant leaving the engine reaches 95C. When the engine is at normal running temperature the fan would not always be on because the coolant has not reached the switch operating temperature.

I would have to disagree with that I think. The fan is there to prevent the engine overheating, so my logic would be that you want to be monitoring the hottest part of the system. Otherwise you could be getting cylinder head damage while the output from the bottom of the radiator is still within limits. So I have my switch in the top hose. However I have to admit that the original switch on the Superspec was built into the bottom of the radiator (it uses a rad from a Rover SD1), so maybe I’m talking rubbish

That was how it was designed by the original owner You are right of course which is why some keen owners fit auxiliary electrically driven water pumps as well. to maintain circulation while the fan is running. I have found by experience that it seems to cool off a bit quicker if the fan stays on for a bit even though it is only the radiator being cooled, but maybe I am imagining it. So it's really personal choice, but if you go to a kit car meet and line up with the others you will find quite a few of them are gently humming to themselves long after the owner has left.

I had an incident where I turned on the override switch and the dash light came on but unknown to me the fan did not turn on. Luckily it was cold day so I didn’t actually need it. So make sure you wire it so it actually tests for the fan running and not just that the switch works. My fan has a permanent 12V feed and a switchable earth and the 12V supply had failed. But the light had a separate 12V supply so came on when the earth switch was made. You also need to decide if you want the fan to run on after the engine has stopped (This is what I have). If so then it is a direct battery to fan feed. If not then the 12V feed needs to come via the ignition switched 12V supply and will then probably have to operate via a relay.

I've put mine in the coolant line. You can buy an aluminium hose fitting with a boss on it to accept a switch.