Jerry Hey wrote:

Question:   If the radiator  was  placed in front of the firewall, how
much space would required between the firewall and rad?  Assume a
generous outlet at the bottom of the firewall.    ---Thanks,  Jerry

The problem with that is the last thing you want to do is heat the
air before it goes through the rad. Every degree difference between
rad temp and ambient air temp is critical to the efficiency of 
the cooling system! EVERY degree!!!

Here is what Power Sport had to do and despite an oversize rad
and oversize inlet it still did not cool on a hot day.

To answer your question it depends on the size of the rad but about
eight to ten inches MIGHT work for airflow but that does not mean
the system will work to cool the engine.

I do not recommend putting the rad behind the engine. If you don't have room
under the engine for one large area thin rad  obliquely mounted use two thin 
large area rads obliquely mounted. One on  each side of the engine.
 
Paul Lamar
 
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