back left. down from #4 plug.

Running straight coolant or straight water is bad either way. Water will run cooler yes, but if you do by chance overheat the water evaporates at a much lower temperature. Mixed with coolant, and the pressure of the cooling system will keep the boiling point higher. 50/50 is your best bet, regardless. And you don't actually want your truck to run too cool, its not good for its components. That mid range on the gauge is between 180 and 200 degrees, this is where you want your truck to run. Colder is not always better. A little hotter is fine, and some vehicles get better mileage when they run slightly hotter. I like the 180 thermostat, because it keeps it at a comfortable ranger. But on the way to Drop Zone I ran the 195 thermostat, and averaged 30 MPG, even with the grades.