Riggerrob, the deep/high hulls of the M3/M4 was not caused by the "diameter" of the radial engine. It caused by the idiotic decision to use a powershaft mounted directly on the engine's nose case. The M18 had the same engine, but it used a gearing so that the power transfer shaft could run horizontally along the floor of the vehicle. A look at respective cutaway drawings easily reveal that. Had the M4 used the M18 setup, the tank would have been substantially lower and lighter (smaller hull=less weight).
The reasoning that the "inaccessibility of lower cylinders" was in practice of any significance is dubious; it defitinely wasn't in the M3/M4/M18. What is more, a BMW 132 rated at 520 or so hp in a tank installation means an engine running much below its capability, thus giving reliable and durable operation (in aircraft the same engine was used with powers exceeding 1000 hp and basically always operated at above 520 hp).
What is more, the Maybach suffered from serious issues like coolant leaks plus had to be considerably derated to keep its life reasonable. In Finnish Air Force maintenance experiences in WW2, air-cooled radials required far less daily maintenance than their liquid-cooled companions.
Honestly speaking, although fine engines in theory, Maybach's WW2 tank engines were in practice quite poor with highish specific fuel consumption, poor torque characteristics and lack of ruggedness (Finnish tank drivers driving the Stug III were warned that its engine does not tolerate rough handling like the previous tank engines (T-26/Vickers) they had operated). A bad feature for a wartime engine.