My understanding is that most of the electric fans you see are from later Fiats. Theoretically it shouldn't matter on which side you put the fan, but you'll have to make sure the blades are curved in the right direction. If it's blowing forward and you have the back side of the radiator sealed it should push hot air into the cabin. Also, you'll have to use the curved outflow tract so most of the flow that doesn't go for heating ends up facing rearward, I would think. I've read that for reverse flow it's best to put a curved scoop under the car facing forward.
I read that to have a real heater you have to install a unit from an 850. My family had a 600D when I was growing up, but we lived in the Los Angeles area, so a heater was never important and I don't remember how well it worked.