Do you know if the Continental Z120 Engine was made in 1946 specifically for the To-20? or was it made prior, and was it used in something else? Also, the engine-block cracking problem that it had, was that from the ones that overheated, or from something else?
I believe the Z120 as well as the later series Z engines were used in other applications. Yes, I believe overheating was a major factor in these blocks cracking. Improvements were made in the cooling systems of later versions of the Z series engines, the Z129, Z134 and Z145. These changes may have reduced cracking but it still remained a problem.
Sadly, poor cooling system maintenance by owners and operators was also a major factor. I do not have specifics but I can tell you I am not surprised when I work on one of these engines and find cracks in the top webs between the cylinders. These are usually not a problem. Cracks at the lower end usually go through the crankshaft journals and these are the ones that allow coolant to leak into the oil.