My fans used to kick in Just before the red on the gauge, When i changed my engine it was the same.
I changed my fan switch, dash sender and put an 82 degree thermostat in and it was still no different.
After all that i cut all the shitty ford connections out of the fan loom and run it up the centre of the rad and over the top (cut out 7inchs of ford loom) and now the fans kick in on the top of the O on NORM.
When doing this the Earth wire looks all black inside so the wire has lost all its goodness still works fine but i will replace with my own direct feed from the battery to power the relay and fuck the ford bit off out of it.