1. Brazilian Blocks are cast by a company named "TUPY" and are marked such. They are identified with cast evolution numbers. #53's are TUPY's. #54 and later evolutions were improved to correct the block issues that the 53's had.
2. Mexican Blocks were made by "Teksid" these blocks to my knowledge never had a problem with cracking, as they are heavier duty by trait, but they did follow the evolutions and got thicker. These blocks do not have the 2 digit "cast number" embossed on the side, instead a series of numbers that are unidentifiable to me. The Mexican blocks were rumored to even be harder to machine do to a harder cast overall, basically better quality in my opinion.
Brazilian Tupys always seem to outnumber Mexican blocks during the ISB's production probably about 8 to 1. So basically if there is a pattern number its Brazilian, if there is a series of small numbers its Mexican.
By the way the Mexican equivalent of a 56 pattern was the Heaviest ISB block of all. This means if you have a 2002 without a #56 on it and instead it has a series of numbers that are smaller and hard to identify, you got the the big boy!!!
Simply, the worry is that the block will crack. These blocks have thinner water jacket walls than other blocks so with coolant cavitation, corrosion, coolant pressure in the block, and torque, they may crack, spilling your coolant. I know from experience that it will not blow up the engine, but you will see that temp gauge rise more often than you'd like.
A crack develops on the right side of the engine, below the freeze plugs, right where the block bends back towards the inside of the block. It is a difficult area to see for auto transmission owners as it is directly behind the transmission heat exchanger. At first everything evaporates so there is no spot under the truck, but you may notice a coolant odor. The leak doesn't occur at idle in most cases because the crack is tight to get through, and doesn't leak until the RPMs of the engine are up, with the water pump causing higher pressure inside the engine block due to the restriction of the thermostat.
The crack will grow over time and you will go through coolant faster. I found that I lost coolant at a slower rate if I let the engine warm up for a while, so the thermostat opened and lessened the pressure inside the engine block.
If you want to see if your engine block could be one of these dreaded "53" blocks, crawl under the front, left side, and look up at the block on the front, directly under the injector pump (by the PS pump). You should see a large wiring harness mounted to the block. Directly below this, just above where the oil pan meets the block you will look for a "53" cast in the block. The numbers will be about 1 inch tall.
I found this in a search.