Garage Heat?

If your garage is 28X36X10, guessing the ceiling height X about 7 gives you approximately 60,000 btu's needed to heat the space. If the "R" value of the insulation is good, that should do it. Personally, if you haven't poured the floor yet, use the floor heat. Wersbo, pex what ever you want to call it. A little pricey to install but real cheap to run. And if you're on the floor, warm as toast.
My two cents. And X 2 on the vent free. Combustion causes a lot of moisture.