Dude, the heater core is not sitting directly on top of the case.

As I said, I built a shroud/stand for it.
Its totally ghetto, but it gets the job done...
There is a 1-1/2" gap between the face of the HC and the case for air to get sucked up into the heatercore. There are 2 air inlet opennings, one as you can see in the picture on the front, and another on the back.