The engine is more than capable of producing enough heat with the correct thermostat even at -40C however at hiway speed and -20C or below I recommend a winter front but make sure it can be opened for milder conditions and you do actually open it or you can partially seize the engine, I know this for sure, lol. At these temps on the hiway the floor becomes super cooled and can draw the heat away very quickly so insulation on the floor is a must have for the winter. If the engine is producing the correct heat and you are not seeing it on the other side of the heater core then if the shut off valve is open all the way the core is plugged internally and disconnecting the hoses and back flushing it with a hose might do the trick or just replace it. For many years I had a second heater in the back for hiway travel and the engine could supply more than enough heat, I recently removed it as I didn't have the need for it anymore, seems we don't get the long extended periods of -30c and below I was used to seeing.

I made my winter front that fits over the 2a grill and attaches with the same 2 screws the badge does. 3 pieces of aluminum one large and 2 small to match the grill, the two side pieces can be folded over opening up either or both sides. I covered it in arctic vinyl so it stays supple and the vinyl is the hinge. I just have a small cabinet door catch to hold it open.