I keep my grids disconnected at the solenoid until below 25 degs. 1 small wire pulled from each solenoid. Reason is, the grids (in my opinion) overwork the alt. I try to remember to reconnect the grids at 25 but have started the Cummins at 19 degrees F cold soaked. It might be a good idea to notify the fire dept. before cranking. Smokes real bad for a half a minute.
I always try to plug in block heat when below 50 degs. Reason, Warm oil moves faster than cold oil.
SFrey,may I suggest disconnecting 1 wire from each solenoid for 1 real cold snap. That way you will know if it will start.
1 battery = 1 failed starter. Starters are "series wound" meaning the slower they turn the more amps they will pull. More amps = more heat.
I just installed the Pro Flo intake and intake spacer from GDP which replaces the factory grid heater, and no problems so far. The intake heater is really restrictive, so for 80 I got a wide open billet spacer, it looks cool too. On the install you completeley remove the wiring and grid heater, I just drove up to Paso Robles and back for work, and got 550 miles out of the tank, which is almost 16 mpg, I was doing 80 most of the time and was only loaded with gear on the way up, but I don't think I got much of a mileage improvement which is what I was hoping for.