So just feedback to the thread. I installed a thermal switch (electro-mechanical thermostat) inline with the fan relay, and it works like a champ (so far).
I ended up epoxy mounting the thermal sensor right to the back of the radiator at the neck to provide a solid connection, using steel reinforced epoxy... seems to work pretty good.
Here's a picture of the final solution (although I'll probably move the switch once I get all the tweaking done.
