So is the current furnace in a utility room or closet?? Is this room close to, or hopefully sharing a wall with the garage? if not, trying to install large enough ductwork with the required cold air return, as well as the hot air supply will be a problem..
So unless the existing furnace is in a room adjacent to the garage, I'd seriously look into a small boiler, pipe the hot water to the furnace, install a water to air heat exchanger in the furnace plenum. You can also use the hot water to heat your domestic hot water, for even greater savings in energy costs..
There are several small boilers available, Leisure Line, Keystoker Kaa2, Harman VF3000,
How big is the house, what climate/part of the country are you in, and what is your current Furnace's BTU rating?