you did a flush meaning you used 100% water to clean out the coolant system now the question is did you drain the block and heater core ? a 350 GM engine takes about 4.5 gallons of liquid . so you should have used 2.5 gallons of antifreeze to 1.75 gallons of distilled water which should be not problem to -40 deg f... the T stat can stick also but I would expect heater to work.