Found this on GT-R register site;
11010/11000-05U*0 - R32 (80 in early cars, 00 in later cars), R33, R34 standard block
11000-24U00 - R33, R34 N1 Block
11000-RRR45 - New Nismo Block
"I suppose the rumor that the R32's are tougher may relate to the early 80 blocks. The other thing is that the metal gets stronger as there is less stress, de-stressing is done by time and/or temperature cycling, and R32 block is older and likley to have been through more temperature cycling.
The N1 block has thicker walls on the cylinders and smaller oil channels to the head, like the tomei items. The Nismo Block is designed for 2.8l conversions it has even thicker walls, thicker top section to the head and maybe revised water channels."