Well, supposedly the mexican block has a higher nickel content which should make it stronger. I believe it also has thicker main webbing since its an older motor (before ford lightened up the block for fuel economy). Some say the nickel doesn't make it stronger, just means you have to run different rings. I do see an aweful lot of people running the Mexican block though, but not sure how high HP they can take. If you build a turbo 5.0, you can get over 600hp if the tune is good. With blower or nitrous, all bets are off.

I would just build a 351, the block is good for about 800hp stock and nowdays its not a whole lot much more money if you're building from scratch.