The rumor was that the Mexican blocks have more nickel in them, making them theoretically stonger. However, the higher amount of nickel is not even worth it. If a block is going to crack, then by God it's gonna go. All of the 302's I've built have been American made, and I've had absolutely no problems with them ever. If you require a stronger than stock block, you'll have to get a main girdle and other such things, but by then you'd better be running a hefty shot of juice and/or boost. Hope this helps!
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch