Baneblade chassis- the wheels / fenders on both sides. The wheel / fender sides are the first things GW has you build up, you know, the sides where the sponsons go and that are eventually joined by the large bottom midsection.
Figure out how big you need the mortars to be and hen build the middle to fit. You get to keep lots of good bits too
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I thought of using 4 basilisk platforms build up, and then linked up on a single platform would be the way to go. You could cut the barrels to make them more mortar like. You could have them a little offset with the wheel sides by having them mounted a little lower like the front portion of the shadowsword to give it more depth. Have a bunch of mutants / traitor guard manning the guns / loading the guns and have a berserker with a whip in the back. = profit.
It's only Str 7 AP 4 Apoc (4) but it is super stying. It is daemonic which helps a lot to keep it firing.