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Chris my point on the hardness needed to prevent failure is it probably block design related. Different blocks have more redundancy in the number of fasteners(2 bolt, 4 bolt,cross bolted mains), the size of the fasteners the other cross supports that all affect the loads on the block in specific areas. Why our Ponchos need a lifter bore brace with big cams.

The Brinell is a hardness test and the engineers can correct me but is an indirect measure of tensile strength of the cast iron.Most are using good cast iron blocks with brinells in the low to mid 200s,as was mentioned the BBC 210-230. .So some of the ads are for the tensile strength not the Brinell number but there is a conversion factor.The IA2 is listed at 235 Brinell and 35,000 tensile strength.I believe a Ford ad for the new Boss block was for an even stronger steel than most of the other aftermarket blocks are, I want to think 40-45,000 tensile.

You can do a search for the actual numbers these blocks in the discussion came up with.

So if the big factories "race blocks" use a higher tensile cast iron than their passenger car blocks I'll bet the engineers had a reason- as Tom V says