Whatever pistons you choose, make sure that the guy measuring them is the same guy machining the block. Measuring with precision tools has a lot to do with feel, and your number might be different than his number. Due to this fact, Micrometers and bore guages are most accurate when used as comparative tools than when used to determine the actual size of something. The number itself makes no difference in this case though...it's the comparison between bore size and piston size that matters, and if the same guy is measuring both and simply comparing the sizes of both, you'll be a lot more likely to hit the mark if you give him both the pistons and the block and call out the clearance spec than if you give him just the block and tell him how big you think the pistons are.
