I considered servitors or space marines on external seats behind the plasma arrays. While this would be very interesting it'd be a bit too Mechanicus for an Astartes tank.

So I went looking for alternatives, and found this little sub-assembly:

It bolts onto the weapon mounting as follows:

I like the way all those exhaust vents match up. 

Ever watch Macross? Remember all those bits where the pod doors open to reveal ALL THE MISSILES!!!!!?

To make it all fit I had to stretch the forward arm of the weapon mount, adding another hydraulic arm. Still some detail to add here, because you will actually see some bits past all the guns.

 I've made some progress on the rest of the tank, believe it or not, with steady progress on the hull sides and rear section. A lot of plasticard and rivets. Oh, and test fitting two belt fed semi-automatic cyclone missile launchers. I'd welcome your thoughts on these?

  
So the final weapon load-out probably looks something like this: