Hmmm...good question.
I'm not certain how the laws apply to this particular scenario. In general, muzzle loading black powder cannons either manufactured on or before 1898, or replicas of the same, and aren't capable of firing "fixed ammunition" are considered antiques and not subject to the GCA or NFA.
HOWEVER...a tank cannon, even a deactivated one, does not fall under this which means it COULD run afoul of federal statutes, even if it remains "deactivated" an is only used to fire muzzle loaded black powder blanks.
I'll leave that to others that may be more knowldegable on this subject.
HOWEVER...you might not have any legal issues if you made this a carbide cannon, only using calcium carbide and water, with a spark source, to make a rather impressively loud noise. You might even go so far as to use a powdered "blank" charge to affect a simulated cloud, such as common flour. Limitations on carbide cannons fall under state/local laws on the amount of calcium carbide the device can use. But the limitations are generally pretty generous, as it doesn't take very much calcium carbide to make noise that's hearing-protection loud.
Calcium carbide, when added to water, generates acetylene gas. Mixed with the oxygen in the air, this makes a very reactive, and LOUD boom. It doesn't take much. Coming from a 76 mm barrel, it could indeed be impressive.