I had a Hunter 380 but have been aboard the 38 (looking for the differences) and I've also had four other Webasto diesel furnaces in my three previous boats and now have the fourth in my present H27. They are good heating devices. We also had one water unit plus the three hot air.
Hoping my memory is correct, my Webasto in my Hunter 380 was in the stern port locker, but underneath the decking. You could still use the locker for storage however, the furnace unit was underneath it. You could service it by removing a panel in the aft cabin bulkhead.
The run for the hot air ducting went down the port side under cabinetry shelves in the aft main cabin, through a hanging locker, behind the head and through the storage locker in the head and then into the main cabin where it came out in between the two lockers under the settee. One advantage to the long run was that all those lockers stayed warm when the furnace was in use. The boat would warm up in about twenty minutes when the furnace was turned on on cold days (40 degrees).
Although I've never used your unit, I would recommend a small fuel filter and a muffler on the exit tube. Webasto furnaces have gotten much quieter in the last few years but I still think making them absolutely quiet when at dock is essential to good neighbor relationships.
Hope this helps.
Les