You could also use a tank heater. We tested some of the soviet vehicles at -65 F. One vehicle had twin what like to be Chrysler flathead engines. They used a mini-boiler that was fueled by gasoline that heated the coolant in both engines and the exhaust off the heater was shrouded around the oil pans to heat the oil.
It had 2 Diehard batteries.
The first start attempt we ran the coolant heater for one hour. Both engines started easily
after the preheat. We made another start when the oil in the oil pan reached -65. When we started the coolant heater it quit heating after 10 minutes of use. The test engineer ask me what he should do. I said turn on your instrumentation and lets make a start. When I hit the starter the first engine started easily and so did the other engine. We never had to change the batteries. Hal