The first unit entry in the second Horus Heresy source book from Forge World, Massacre, is the Mortis Dreadnought. The Mortis is one of the few units available to the legions that has the sky fire rule. But to activate it, the Mortis must remain stationary and not run during its turn. The added bonus is to gain interceptor as well. Therefore this is a unit that probably wants to be set up in the first turn in a similar location to heavy support squads -- with a good line of sight to most of the battlefield, and the airspace above it.
The other principle differences between regular legion dreadnoughts and the Mortis pattern is that fact that it must wield two of the same ranged weapons, and that they are not fielded in talons (i.e. one force organisation chart slot only gives the controlling player one dreadnought, rather than the option of having more).
Below are a couple of builds to consider.
Legion Mortis Dreadnought, 2 twin-linked missile launchers, havoc launcher (160 points)
This is a great choice and gives a tremendous amount of tactical flexibility between the krak and frag missiles, plus the ability to take down fliers as required.
Legion Mortis Dreadnought, 2 twin-linked las cannons, armoured ceramite, extra armour (185)
Beefed up with the extra armour and the armoured ceramite in case of outflanking melts-toting enemies, this mortis is intended to sit in one spot all game at take down the heavy vehicles and fliers wherever they move to.
Legion Mortis Dreadnought, 2 twin-linked multi-meltas, armoured ceramite, extra armour, 2 hunter-killer missiles (175 points)
Unlike the above configurations, this is one that will probably require moving to obtain a good position given the range of the weapons. The hunter killer missiles are there to use as it moves up to its target.
Legion Mortis Dreadnought, 2 twin-linked auto cannons (135 points)
A basic, dakka orientated dreadnought with a minor capability to take down low AP fliers (from the rear, of course!). Cheap and cheerful, but I think I'd prefer one of the above to take full advantage of the helical targeting array. But note, this is actually cheaper than a similarly armed legion dreadnought.