Just been looking at pics on the Forge World site, and I cannot work out which Malcador variant you built this from (I'm assuming 'battle cannon' as you have used one on the new turret).
I have not seen the kit up close but they all look as though the issued 'turret' (bunker more like) is built into the hull top. Did you have to cut, or where the parts separate?
New turret reminds me of a scaled down Macharius turret, was that the inspiration?