The Australian cruiser stuff is easy, they did it exactly the same way the US got a 69" turret ring into the M4. The M4 would seem to have a hull that has an internal clearance of 60", so a 69" turret ring won't fit, except the M4 has sponsons that completely overhang the tracks and it is not a problem. The Sentinel, I absolutely agree, a 70" turret ring is not going to fit, not because side to side but because with the fighting compartment roof dropped like it is to reduce the height of the tank, fore and aft there's not enough room. If the AC1 is Australia's M3 the AC4 is more akin to something like the T20 or T23 it shares the tracks and suspension of the earlier tank and a few other bits but not much else, and on the AC4, and its predecessors for that matter, the air vent's louvres partially overhang the tracks, this means the top of the hull is much wider than the the 64" internal clearance the hull might otherwise limit you to. The turret basket, if one was to be fitted, just needs to initially taper inwards at a sufficient rate that its diameter is less than 64" before it clears the bottom of the hull side air vents, or a inverted conic section needs to be moulded into the hull sides, or something similar.