It's a Penitent Egnine "frenzied driver". Sisters of Battle (Whichhunters) have those as Heavy Support. Though I had to widen the whole body to fit the driver in.
http://kofler.dot.at/40k/units/Witch_Hunter_Penitent_Engine.gif
I play it as my Venerable dread if I need one, I know it doesn't seem very well protected and armored... But since it's so old and ancient, heavy shields and the Emperors will is protecting it.
I greenstuffed morecable and his arms going into the Dredd. The top armor is the normal one, but i flipped it only to realize the shape was perfect but slightly smaller than needed, So I filled the gaps in the shoulders with Greenstuff and plasticard (old credit, phone, gaming and other cards work out great and are really easy to get and is basically plasticard). The top is made up from two gunbatteries from Imperial Battle Fleet Gothic. Almost a perfect fit on the top. :)