Thats another option I was looking at. I was actually thinking of using 3 of these inline in each of the ducts and have them sucking air through the heater core rather then blowing into the front of it. I might actually change tact and go with this option. I think it will deliver better air flow results for the same price. And might mean the heater box can be a lot smaller.
Thinking about it.....if these were in line, they could actually go anywhere in the duct which will give a bit more flexability.
The only issue is the noise....blowers are usually not the quietest things in the world!!