Here are a pair of Imperial Guard Heavy Tanks from Simon’s collection.
These conversions were made, probably about ten years ago, so way before the Death Korps of Krieg models found their way into Epic Armageddon and Forge World.
I am pretty sure that these are Hydra hulls, as for the turret, well if you look at this picture of one of my Ork tanks from the same era, I think you will be able to guess!
From a fluff perspective, it can be easily imagined that an Ork army overtook an Imperial Guard tank factory and utilised many of the components including the turrets on their own tanks and battlewagons.
If I remember correctly, from Simon’s perspective I think it was because he had all the Ork vehicles from the Epic 40000 boxed set that he decided to use the Ork turrets.