"We are purchasing
a 2004 Maxim 380 with an approx. displacement of 6 metric tons."
I'm inclined to think that this is an interesting calculation, but probably not necessary.
There are bunches of different measures called "displacement," and they aren't the same thing as dry dead weight. In addition, starting with such a rough measure prevents your calculating something more precise.
Do you intend an unusual task for the boat, or are you planning on usual cruising for the two of you? The designer
figured in expected load when determining the hull
dimensions, so if you're not using the boat to smuggle gold you are good to go.
Distribution: You can break a 1,000 foot tanker by filling the fore and aft tanks
but not the midship ones. That said, I doubt you are going to notice a change in handling of the boat unless you put all the cargo weight in one end. Sure, divide it by hulls and tend toward weight in the middle rather than each end, and again you should be good to go.
Perhaps think of it in terms of your moving about in the boat. Standing you up at the forestay or at the helm
makes little difference.