[UltraMar]

Im not sure you should go with a twin linked Lasscannon

The best set up for a dreadnought, is Assault Cannon and His Claw, Check this out, the Assaut cannon is strength 6 rending. So against a vehicle if you roll your 4 dice to hit and they all hit on 3's. Lets say your trying to pen side armor 12 of a vindicator you would need 6's to penetrate, but if get at least two 6's then you roll both of them and whatever the total is will add on to ur strength pretty much asssuring that you now have 4 pening hits, now on 4's the vindicator will be destryed, you have A LOT better chances of destroying it with an assault cannon than a LAsscannon.