Shrouding
 

I planning to buy the D-teks heater core and I'm planning to do as Joe and pull the air from the front and out of side (or the other way), so that the air dosen't heat up the system.

Should I place the fan as far from the radiator as possible?

Where should I decrese the shroud from the heater core's areal to 120mm (fan)?

about 5-6cm will probably be enough....

hehe, ive got 3.5 inches between my rad and my 172mm fan... a bit overkill, I think if youve got an inch between the two your safe.

It isn't that the farther the fan is away from the radiator, the better. What you are trying to do is to get air to be pumped to theh "dead spot" on the fan. The dead spot is right behind the motor where the fan blades don't spin over. So an inch is generally enough to let the air spread to the dead spot. Within reason, any other increase in shroud length should not decrease performance as long as it is well shrouded.
-Kevin

Kev, not only to the dead spot, but generally get air flowing throughout the entire area of the radiator. If your radiator is 150mm x 150mm, you will only need an inch or so if you are using a 120mm fan. Whereas if your rad is 160mm x 200mm, and you are using a 120mm fan, your shroud will need to be a bit deeper, 2 or 3 inches

Obviously every place where the fan blades are not directly over would be a dead spot.