Having the cap off and only using water were the culprits. Use a 50/50 mix and a radiator cap. Left alone, water will boil at 212 which I'm sure the heads saw consistently. As soon as you shut it off, the water stopped moving and rested against the screaming hot metal of the engine. Bingo... boiling. Pressure also prevents boiling which wasn't there without the cap.
Mix 50/50, use a 15 lb cap, and you should be fine. If you're not, ditch that electric fan. A belt-driven fan takes as much as 10-15 hp to run. You can't replace that kind of airflow with a 3/4 hp electric motor, especially because it draws HP, too when the alternator tries to keep up.
Once you have all of that in place, don't worry about the actual temperature unless its going past 220 or 230. As long as you don't have boiling you're fine.