The mexican blocks have a higher nickel content than other 289/302 blocks, but as far as I know you can only overbore them as much as any other 289/302 block.
To me, the major appealing thing about the mexican block are the main caps, which are comparable in thickeness to the 289 K Code main caps and thicker than standard 289/302 main caps.
Tear it down and have it sonic checked. The thinnest walled cylinder will determine how large of bore you can go... which is determined by the machinist of your choice.