Quoting Gottfried van der Horst
nice try but there was already something like it it was called the Brummbar it was the Jagd version of the kingtiger it had 380mm cannon with only ten rounds.
Correction- the Brummbär, or Sturmpanzer IV, was actually an assault gun from earlier in the war, built on a Panzer IV chassis and used a 150mm gun. The Jagdtiger was the last German tank hunter of WWII.