You would also need an expansion tank. Vodka is flammable and the worse thing to use an antifreeze in something like this. As said earlier, polypropylene glycol is what you would need to use. 60 GPM through 1/2" pipe and a auto rad would require a pretty strong pump, so you would be using more then just 500 watts. You also need to add some sort of aquastat device to limit the amount of heat in the stove.
Wood fired boilers (and that is what you are making) have a tendency to rupture if there isn't sufficient control over the heat. And it will probably take you longer to heat the area than use the area. It would be much more efficient, and perhaps economical to get an LP heater of some sort. The hanging heaters work quite well and LP is just about 1/3 the cost of kerosene....at least around here. I do have an LP heater which is Dayton and it is very compact and half as loud as the normal torpedo heaters. But at $4.50 a gallon I am using it sparingly.
Do you want it your way or the right way?
why would it rupture? really I dont see the water getting super hot with how much it be flowing. Ill have a tempature gauge on it to detect the liquids temperature as it enters and exits the garage. I want to stay away from any type of electric or gas style heater due to the running cost. Wood i can get for free pretty much. Just dont have room in the garage for a wood burner. Thinking of doing a wood burner with a sealed chamber around it and just run air around it heat the air and have it go into the garage. but that involves running like 2 8 inch lines or so and alot more work. this liquid idea seemed better