The Russian SU and ISU series of vehicles had a limited traverse of the main gun of twelve degrees, much the same way as the German Jagpanzer IV/L48 and L70. This vehicle was slow, and only carries 20 rounds of ammunition. Resupplying the monster took nearly 40 minutes because of the weight of the rounds, and reloading in combat took nearly 30 seconds, longer if AP rounds were being used as they were a two-part affair, similar to naval ordnance. The optical system was also terrible. For close-quarter (under 900 meters), you used one sight, for anything over, up to 3000 meters, you used another site.
This is not the type of vehicle you'll be rolling to pork a base...although in a fight, it's survival rate would be fantastic, especially against anything currently in the game.
If you're looking for mobile artillery, I would suggest the Hummel and the M7 GMC. Both can be based off existing models (the PzIV and Sherman chassis), saving a ton of work on the coding side for movement, hit boxes, internal compartments (driver, etc). Both have their disadvantages, but the ability to stand-off and pork a base from long range (outside the arc of predatory GVs like tanks) would offer another possibility.
Jeff