I have the deluxe heater in my 55 Cameo, and it was warm enough that I used the truck daily in 1968-70 in the coldest of the prairie winters with temps down to -35 F. At that time, I was running a 283 in the truck.
The one thing that I did to help keep the engine a little warmer, was to put some cardboard in front of the radiator so that the engine would run a little warmer.