My father in law and I have a hanger where I keep the Camara and a few other vehicles, the area where the heat escapes is the main door and the ceiling. Both are insulated but where the door opens ( on the sides and top and bottom) there is rubber there but it isn't a strong air tight seal. Heat rises obviously so if you have any areas in the ceiling where heat can escape I would recommend insulating as best you can to fill in any gaps.
So with all that said, I would recommend an install that pushes all the hotest heat away from the door, and directed down. You will have some heat that will escape through the door and ceiling but without seeing a picture of your garage this would be my best advice based on my experience.
FYI we also have some ceiling fans in our hanger and when we crank up the heat in the winter we use those to push the hot air down to the floor, this seems to raise the temp throughout the hanger about 5 to 10 degrees.
We keep the thermostat set at 40 degrees during the winter, and if we are going to be there a while we may crank it up to 50. When you come in from temps in the teens or below outside to 40 or 50 it feels awesome.