OSH, before you rush out and spend a fortune on liquid cooled heads and other complex fixes, try contacting builders who have installed the six in tightly cowled aircraft such as the Sonex.
Many of the Sonex builders in the USA favour the six cylinder Jabiru over the few other options because it really turns the aircraft into a great little gadget. But they all seem to do without the standard Jabiru cooling plenum, instead either using baffles marketed by the kit supplier, or making their own. Some ditch the standard Bing carby as well in favour of other options spruiked by Sonex.
My Sonex only runs the four cylinder Jabiru, so I won't muddy the waters with comments on that installation other than to say so far, so good.