re:  And one minus is heating...To heat a little Porsche is one thing,
but heating a huge T3?

IMHO best bet when installing a Porsche boxer is to do it in aircooled
Vanagon ('80-'83) which probably already has gas heater.  See
http://www.msdsinc.com/vanagon_ecs.htm

 Nevertheless the question posed by this writer has to refer to the
Porsche's cooling fan, which, like the VW's is, I believe, designed to
cool the motor and is not enlarged or anything with respect to the
vehicle interior size.  I expect cooling requirements of a 7000 rpm,
6-cylinder, 3-liter, high performance motor to be greater than those
of a 2-liter VW.