I've broken a couple sets and they were motors in the 130-140hp range. I know of others in that range that have broken as well.
They like to crack from the right rear cylinder stud of the front cylinder, down to the timing plug, and then up to the right front cylinder stud of the rear cylinder. Usually the first symptom is either an oil leak or more commonly, a blown head gasket because the stud loosens up due to the crack.
I'd say at anything over 120hp you're probably at some risk, but over 130 you're definitely pushing it.
That said, I have yet to see a set of cases break when they have cast iron cylinders sitting on top of them. Not to say it can't happen, but I've seen cast iron cylinder 88's up to 153rwhp that were holding on stock cases.
One of the things we do to mitigate it is to build 82ci motors with stock stroke and 3.700 bore. With that bore size we don't have to bore the cases as big, and they don't crack as easy. We've touched 140hp on this configuration running gasoline, normally aspirated, without hurting the cases, so it's a good setup. Good for racing classes with a 1350cc limit or a minimum weight based on displacement. We also have done 3.600 and 3.625 stroke/3.813 bore 82 inchers. They're capable of a little more than the small bore/long stroke arrangement but not as much as you'd think.
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