9. Turtle MSV ( Large. ) : Basically this vehicle http://www.3vwargames.com/galleries/gall_ww2/sdfz2.jpg. Use duel project 2206 truck engine and add a rotating platform inside able to turn 360 degrees to mount a heavy weapon on. Give it 1.5cm thick slopped armor on the front 5mm the rest. ( AA gun, Indirect artillery and such. ) Include a tow bar on the rear.
9.1 Counter proposal: Mount the new 120mm gun onto it, and strip all armor, and turret traverse, leaving only the feed mechanism and elevation intact. It should be firing backwards and if needed cut the barrel short. Keep a limited traverse like other guns of the era. In other words, a self-propelled gun.
It's, partially, a question of what you want your basis to be on. Historically, the self-propelled anti-air guns grew from modified personnel carriers, like half-tracks or trucks. See, for example, the Sdkfz 251/17 or the M45 quadmount. Contrary to that, most self-propelled artillery was mounted on outdated tank chassis, like the Wespe or the Priest. The reason for that, as far as I can say, is the different weight and strain on the systems. For example, a 40mm AA gun in our game weighs two tons - the 80mm artillery piece seven point five!
This means that the same system would either be underpowered for the artillery or overpowered for the AA gun. Additionally, firing the 80mm artillery gun will produce much more recoil than the 40mm AA gun, possibly enough to topple the vehicle if not fired in front or to the back. (This assumes fixed weights. Plus turrets, it's 5t vs 25t)
On the advantages of mobile vs fixed AA:
Mostly, cost. Basically, we can assume the cost of a mobile AA vehicle to be bigger than 9 construction points (truck + 40mm gun). This is a pretty conservative estimate (no armour, only one engine and so on), and I'd expect more like 15 or so cost. While the fixed one requires fortifications, it still has at least an advantage of 2 - two fixed AA guns vs one mobile.
Additionally, it can be better fortificated. Concrete is cheap, it doesn't need to move, and so on. So, for defense, use the fixed one.
Mobile AA guns can, and are, more effective when the others cannot be brought to bear, for example in a land offensive. Which is pretty much what we can count on not to happen for now.
Edit: Clarification: What I mean by fortification is to dig a hole and put the gun in there. You restrict your firing arc, but only direct hits will not affect the gun. It's a similar principle as foxholes. Something like this:
http://en.wikipedia.org/wiki/File:Pillbox_Type_23,_St_Martin%27s_Battery,_Westen_Heights,_Dover_%28rear%29.JPG