50 below this morning with the windchill and 37 below tonight with a windchill of 47 below. But the old CJ with the mods I made has great heat even at these temps but it does need to be plugged in to start.

Old Green, hey? I recently found myself living in near-Canadian-like weather conditions, minus 30 degrees F. Three year old battery must have been frozen, won't turn the starter! So, you apparently have a block heater? In your opinion, for -30 degrees F. temperatures, which would be the best application, a block heater or a battery heater?
My experience, limited, of course, has been that if the Jeep engine starts, it will eventually warm the engine coolant and engine oil. Fortunately, I had a 750 CCA Orbital battery to replace the original 600 CCA Jeep battery, and got to work although the Jeep was mighty cold even though I warmed the engine for about 10 minutes before heading out.

Definitly the block heater, without it your dead in the water. Plus I have a spare yellow top optima behind the pass seat that is used for the winch and the stereo that I can run booster cables off of. Don,t do much wheeling in these temps though, it,s a little dangerous if you have a breakdown.