[Chuck Gallagher]

Jeff:
Engine blocks are like home made cakes from an oven. The quality really depends on the ingredients, and the maker, and the available controls as to how well they turn out..
Two identical (brand) blocks from different foundries, both following the same prints, are not usually going to be the same. A block with a higher carbon content and precision casting core control could be more likely to hold the cylinder walls shape (round and straight) and have stronger main webs then a much thicker block that had lower carbon in the iron and poorer control of the casting cores.
High volume production blocks generally are not going to result in castings that will hold their shape at the horsepower levels (600+) racers want. This is why todays purpose designed racing blocks are, in the majority of cases, cast using compacted graphite which is more robust but also more expensive.
So before investing in machining any recycled production block, I would recommend that you test the block for hardness of the material and the amount of core shift that may be present and if a new racing equivalent is available from a reliable source.
You don't want to end up with an engine with a cracked main web or seven perfect and one bad cylinder.
Chuck Gallagher