Are all 340 blocks the same?

OK, I know its probably been discussed but I did a search and could not find the info. Anyway with that said I'd like to know if all 340 blocks are the same or if there's a particular casting number or year that's better or is there any years to avoid. I'm planning a small block build and I'm going to start looking for a good used and not used up 340 block. I don't need any info on cranks or any other parts, only the block.