I would feel safer starting the old motor every 2 or 3 weeks and letting it come up to temperature rather than adding who knows what to the
oil and gas. Fill the tank before putting it away. I keep a 60 watt light bulb going in the cockpit (it's on the brake pedal on a clip - Home Depot). By keeping the wiring warmer than the surrounding air it prevents condensation.
Loosening the valve springs is unnecessary. The springs do not fatigue when sitting still, which is what they do 99% of the time even in warm weather. I also keep a light bulb going under the crankcase to keep the
oil warm. That way when I do start it, the
oil moves. Use 5W-30
synthetic. It will flow in a hurry in the cold weather.
If you store it on the tires, roll the car to a different position every so often to prevent flatspotting. If you drive it in the cold cover the oil cooler (foam or sponge) and cover lower 1/3 of the radiator so that the returning coolant is warmer than the ambient.
Do not cover the car with plastic, it traps moisture!
Bob