Might want to read up a bit from both the International Residential Code and the International Fire Code...
R302.5.2 Duct penetration. Ducts in the garage and ducts penetrating the walls or ceilings separating the dwelling from the garage shall be constructed of a minimum No. 26 gage (0.48 mm) sheet steel or other approved material and shall have no openings into the garage.
M1601.6 Independent garage HVAC systems. Furnaces and air-handling systems that supply air to living spaces shall not supply air to or return air from a garage.
Table R402.4.1.1 requires: “Air sealing shall be provided between the garage and conditioned spaces.”
If this was new construction and you wanted to heat this "garage" with duct work and use it as a garage it would be a big no no. If you wanted to heat it as a "shop" or "garage" the the insulation of the foundation would be an issue since it would need permiter insulation. If you wanted to heat it as a shop with ductwork for new construction...no problem if insulated properly.
If you wanted to go to the city to convert your garage to a shop with a permit you could. "Change of use" permits typically apply to an entire building, for example changing an old church to a home, or a home to a private school but not always. Then, zoning and codes reqm't kick in for the different use. Now, If the city looks at your shop as a living space now, be careful you don't have a zoning and setback issues. Some cities will allow garages closer to property lines since they are not a livable area, changing it to a shop might change that and require a minor variance....yuck.
Get a portable heater... 10 btu per sq ft of garage... Spend a grand and buy a 10,000btu construction heater... Should heat up your garage in 30 min... And you won't have to worry about suffocating your family.