Lets see if i can make it clearer to help,
On the X models unit inside the boat under the gas tank area aimed forward to to rear to keep sailing lean from affecting its working. with the exaust port coming out the back of the boat right in the area where the wiring and steering come out in the rudder area. Small air intake to fuel burn through can be left inside cabin or vented out the back too. The diesel fuel tank is then mounted in the gas tank area or “hole” with a very small sealed hole where the fuel goes down from the it tank down into the cabin to the unit. The fuel magnetic pump can be mounted in the cabin or outside in the tank area. The fused power wires can then be run to the steering guage area, you can hook it to the gauge power or follow the boat wiring to your batteries itself.
For the 26m you can do the above or mount it up further forward up on either of the porta-potty walls. But keep in mind the fumes my blow back into the cockpit area instead of blowing out the rear of the boat. That is why most boat have engine fumes going out the stern.