My KK efficiently sucks in whatever fumes are outdoors-- smoke, gas, whatever... I can smell them easily through the blower. Perhaps your leak is not at the heater core, but in your radiator? Does it do it all the time, or just when the engine is hot? Is the smell stronger with the heat on? Any drops of coolant on the driveway? How is your radiator cap? Just some thoughts...
I have heard that Stop Leak is almost always a bad idea. It will gum up your whole cooling system and lead to needing to replace more than just what is broken right now.
2008 Liberty Sport 4x4 Automatic