I managed to start my 54 Meteor with the 255 Flathead (Canada only that year) at -20C (-4F) last week.
The engine is in pretty good running condition, most cylinders are 95 to 100 psi, one dropped to 70 (might be a sticking valve) last summer, but still runs nice.
Last winter I could not start it at all unless I put an oil pan heater on it overnight, and a trickle charger. I lapped the valves last spring and now the compression is pretty even all the way across (other than the one cylinder).
To start it last week, I pumped the gas three times, hit the starter and the old 6 volt turned over REAL slow. I was lightly pumping the throttle to about 25% and it fired in less than 10 seconds. I held my foot on the gas maybe five more seconds and it smoothed out to a nice idle.
I'm also pretty impressed with how well the heater works. You can feel the pass side rad hose is cooler than the drivers side as the heater is actually sucking a fair bit of heat out of the system. Drove it around the block and it kept the windshield clear. Amazing, I just love this old car.
One regret I have was that I used to have a headbolt heater, it was on a 53 Pontiac I sold after I bought the 54 Meteor last year. Didn't even check to see if it would have fit the flathead. Dang.