Hi Peter,
We fitted our heater at the aft end of the cockpit locker on our B34 which is the owners version. This position has worked well as you can sit inside the locker to service the heater and it is aft of the locker base board so does not restrict the stowage space in the locker.
Best advice i can give you is to set up the air inlet to the heater so that it draws air from inside the boat. Ours is set behind the electrics cabinet on the starboard side. The fact that you are sucking air from the boat at the same rate that you are pumping the hot air back in improves the circulation and the inlet air has already been lifted to a higher temperature than outside. We also find that by sucking air from the boat she has remained very dry and we have no internal corrosion problems on the electrics.