Gary,
<< A problem with a plenum is that the space above the front cylinders
is very limited and it...>>
That space is indeed very limited.  You have to complete your vertical
expansion long before you get to the first cylinders.  On the 320/360 it
can be done by the time you get to the forward engine baffle that reside
behind the flywheel.

<< Eliminating the leakage is a good thing, but a carefully fitted
rubber flap against the cowl will do that. >>
You might find NASA report 3405, "An Experimental Investigation of the
Aerodynamics and Cooling of a Horizontally-Opposed Air-Cooled Aircraft
Engine Installation"  interesting reading.  It is a reference in the
cooling spreadsheet brought to our attention by Jerry Fisher.  The
magnitude of leakage past the stock baffle system in Aztec they used is
mind boggling.  
The report is available at
http://ntrs.nasa.gov
Chris Zavatson