Be careful - by that I mean be very judicious - about blocking off the airflow at the front of the car.

Although blocking the airflow into the engine compartment will preserve heat, it might easily preserve too much heat - by this I mean you might get localized overheating within the engine compartment caused by too little airflow.

I doubt if you need to worry about blocking off the 'radiator' per se, because your vehicle will have a thermostat in the coolant system that prevents the coolant from flowing through the radiator until it reaches the design operating temperature. Only then does the thermostat open and let the coolant circulate.

You might want to investigate and see if the manufacturer of your vehicle specifies a thermostat with a higher opening temperature for use in extremely cold environments. If the engine is 'conventional' (by that I mean not too many electronics in it), you can probably increase passenger cabin comfort (the amount of heat available from the cabin heating system) by installing a thermostat that opens at a higher temperature.

In Western Canada, where temperatures sometimes hit -40 in the winter, it is common for vehicles to be fitted with a quilted pad called a 'winterfront'. The winterfront fits over the grille and reduces (but does not completely obstruct) the amount of air entering the engine area. The primary benefit of the winterfront is a more rapid engine warm-up if you start the vehicle and then begin driving before the engine warms up to operating temperature. They are of minimal benefit once the engine has reached operating temperature, and may actually cause damage (this due to localized hot spots within the engine compartment) if they are used at anything less than highway speeds.

So... investigate this, but like I said, be judicious. Don't just block everything off.