This is a Macharius Standard/Vanquisher turret on the Leman Russ hull. It fits, somewhat, but is comically oversized for the hull.
My Macharius' guns have been replaced with something having a bit more....character. The Vulcan's turret is a bit wider than the standard. This tank is actually fitted with the turret for the Leman Russ Incinerator, so the turret ring is actually the same.
The rear bustle of the turret hangs over the engine, the sides of the turret are over the tracks, and the beginning of the mantlet sits over the downward slope of the hull itself.