I tried a 16 inch fan on my 350 three core radiator without a shroud and it would not keep the temperature below boiling.
I also would not use smaller than a 40 amp relay.
Here's a diagram of wiring up a fan with a toggle override and a operating light if you wanted one. The grounded side of the temperature sensor would trigger the relay and the ignition switch would power the positive side of the coil. You would have to figure out the battery power and the fan power and the normally closed terminals, which I think you have done.
