We have the howitzer from gravity cat. Its close enough
couldn't find any usable Anti-tank guns that fit the period (and could actually cause decent damage to these http://forum.blockland.us/index.php?topic=233440.0)
If I knew how to change the howitzers shell damage or the vehicles health myself I would but I don't.
i believe you can just change the server.cs file
the hetzer in the tank pack is an antitank tank
to change the damage, you just need to open the .cs file containing most of the code, look for a value that says the damage, and change it