A tank is judged by several attributes. Mobility, armor and firepower.
A Priest is an artillery piece meant for indirect fire, a support role. If the Priest does end up in this game wouldn't you need a 'land mode', like in the navy ships, to utilize it's range?
The caliber of the gun does not automatically make it better. The Panther had a 75mm long-barreled gun, it was the long barrel and high velocity and bigger shell charge that made the gun superior, not the caliber. However, my intent is not to troll, the m4 105 should be in the game, given destroying buildings and stuff is a key component to the game. The 76mm M4 had a long barrel, which is where the power came from.
That being said, as semp suggested, there's a real good reason why the gun is still used today.
On the topic of artillery peices, we have one currently called the 'walking stuka', the SDK with the rockets on that side... the rockets that you can't adjust angles with. Why don't we start by fixing that?