The best tried and trued method of determining how much a block can be safely bored, is to have it sonic checked. As a rule, the Mexican blocks have always been considered a better, thicker casting, and more desireable for HP usage. But I have seen some of those that wouldn't be thick enough to safely bore to .060 over either. Most non-Mexican blocks will not be thick enough to bore .100, and would definately need to be filled if they are thick enough, as they are just barely thick enough at best.
