Neil:

In the original Porsche 911s, the heater flaps
either sent the hot air up to the passengers or
just dumped it there right forward of the engine
at the heater jct boxes.  So routing (most of) the
hot air to behind the engine should be fine.

The engine cooling is from the fan pushing
cooler air from the engine compartment over the
engine fins.  Hence the importance of the seal
around the perimeter of the engine; if that seal
isn't good, the hot air being dumped underneath
will come up into the engine compartment and
screw up your cooling.  Ask me how I know.  ;-)

                        Richard A Jones
                        Boulder, Colorado